src/gromacs/gmxlib/ewald_util.c

   1 /*
   2  * This file is part of the GROMACS molecular simulation package.
   3  *
   4  * Copyright (c) 1991-2000, University of Groningen, The Netherlands.
   5  * Copyright (c) 2001-2004, The GROMACS development team.
   6  * Copyright (c) 2013,2014, by the GROMACS development team, led by
   7  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
   8  * and including many others, as listed in the AUTHORS file in the
   9  * top-level source directory and at http://www.gromacs.org.
  10  *
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  13  * as published by the Free Software Foundation; either version 2.1
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  37 #include "gmxpre.h"
  38
  39 #include <math.h>
  40 #include <stdio.h>
  41
  42 #include "gromacs/legacyheaders/coulomb.h"
  43 #include "gromacs/legacyheaders/macros.h"
  44 #include "gromacs/legacyheaders/names.h"
  45 #include "gromacs/legacyheaders/txtdump.h"
  46 #include "gromacs/legacyheaders/typedefs.h"
  47 #include "gromacs/legacyheaders/types/commrec.h"
  48 #include "gromacs/math/units.h"
  49 #include "gromacs/math/utilities.h"
  50 #include "gromacs/math/vec.h"
  51 #include "gromacs/utility/futil.h"
  52 #include "gromacs/utility/smalloc.h"
  53
  54 real calc_ewaldcoeff_q(real rc, real dtol)
  55 {
  56     real x = 5, low, high;
  57     int  n, i = 0;
  58
  59
  60     do
  61     {
  62         i++;
  63         x *= 2;
  64     }
  65     while (gmx_erfc(x*rc) > dtol);
  66
  67     n    = i+60; /* search tolerance is 2^-60 */
  68     low  = 0;
  69     high = x;
  70     for (i = 0; i < n; i++)
  71     {
  72         x = (low+high)/2;
  73         if (gmx_erfc(x*rc) > dtol)
  74         {
  75             low = x;
  76         }
  77         else
  78         {
  79             high = x;
  80         }
  81     }
  82     return x;
  83 }
  84
  85 static real ewald_function_lj(real x, real rc)
  86 {
  87     real xrc, xrc2, xrc4, factor;
  88     xrc  = x*rc;
  89     xrc2 = xrc*xrc;
  90     xrc4 = xrc2*xrc2;
  91 #ifdef GMX_DOUBLE
  92     factor = exp(-xrc2)*(1 + xrc2 + xrc4/2.0);
  93 #else
  94     factor = expf(-xrc2)*(1 + xrc2 + xrc4/2.0);
  95 #endif
  96
  97     return factor;
  98 }
  99
 100 real calc_ewaldcoeff_lj(real rc, real dtol)
 101 {
 102     real x = 5, low, high;
 103     int  n, i = 0;
 104
 105     do
 106     {
 107         i++;
 108         x *= 2.0;
 109     }
 110     while (ewald_function_lj(x, rc) > dtol);
 111
 112     n    = i + 60; /* search tolerance is 2^-60 */
 113     low  = 0;
 114     high = x;
 115     for (i = 0; i < n; ++i)
 116     {
 117         x = (low + high) / 2.0;
 118         if (ewald_function_lj(x, rc) > dtol)
 119         {
 120             low = x;
 121         }
 122         else
 123         {
 124             high = x;
 125         }
 126     }
 127     return x;
 128 }
 129
 130 void ewald_LRcorrection(int start, int end,
 131                         t_commrec *cr, int thread, t_forcerec *fr,
 132                         real *chargeA, real *chargeB,
 133                         real *C6A, real *C6B,
 134                         real *sigmaA, real *sigmaB,
 135                         real *sigma3A, real *sigma3B,
 136                         gmx_bool calc_excl_corr,
 137                         t_blocka *excl, rvec x[],
 138                         matrix box, rvec mu_tot[],
 139                         int ewald_geometry, real epsilon_surface,
 140                         rvec *f, tensor vir_q, tensor vir_lj,
 141                         real *Vcorr_q, real *Vcorr_lj,
 142                         real lambda_q, real lambda_lj,
 143                         real *dvdlambda_q, real *dvdlambda_lj)
 144 {
 145     int         i, i1, i2, j, k, m, iv, jv, q;
 146     atom_id    *AA;
 147     double      Vexcl_q, dvdl_excl_q, dvdl_excl_lj; /* Necessary for precision */
 148     double      Vexcl_lj;
 149     real        one_4pi_eps;
 150     real        v, vc, qiA, qiB, dr2, rinv, enercorr;
 151     real        Vself_q[2], Vself_lj[2], Vdipole[2], rinv2, ewc_q = fr->ewaldcoeff_q, ewcdr;
 152     real        ewc_lj = fr->ewaldcoeff_lj, ewc_lj2 = ewc_lj * ewc_lj;
 153     real        c6Ai   = 0, c6Bi = 0, c6A = 0, c6B = 0, ewcdr2, ewcdr4, c6L = 0, rinv6;
 154     rvec        df, dx, mutot[2], dipcorrA, dipcorrB;
 155     tensor      dxdf_q, dxdf_lj;
 156     real        vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 157     real        L1_q, L1_lj, dipole_coeff, qqA, qqB, qqL, vr0_q, vr0_lj = 0;
 158     gmx_bool    bFreeEnergy  = (chargeB != NULL);
 159     gmx_bool    bMolPBC      = fr->bMolPBC;
 160     gmx_bool    bDoingLBRule = (fr->ljpme_combination_rule == eljpmeLB);
 161
 162     /* This routine can be made faster by using tables instead of analytical interactions
 163      * However, that requires a thorough verification that they are correct in all cases.
 164      */
 165
 166     one_4pi_eps   = ONE_4PI_EPS0/fr->epsilon_r;
 167     vr0_q         = ewc_q*M_2_SQRTPI;
 168     if (EVDW_PME(fr->vdwtype))
 169     {
 170         vr0_lj    = -pow(ewc_lj, 6)/6.0;
 171     }
 172
 173     AA           = excl->a;
 174     Vexcl_q      = 0;
 175     Vexcl_lj     = 0;
 176     dvdl_excl_q  = 0;
 177     dvdl_excl_lj = 0;
 178     Vdipole[0]   = 0;
 179     Vdipole[1]   = 0;
 180     L1_q         = 1.0-lambda_q;
 181     L1_lj        = 1.0-lambda_lj;
 182     /* Note that we have to transform back to gromacs units, since
 183      * mu_tot contains the dipole in debye units (for output).
 184      */
 185     for (i = 0; (i < DIM); i++)
 186     {
 187         mutot[0][i] = mu_tot[0][i]*DEBYE2ENM;
 188         mutot[1][i] = mu_tot[1][i]*DEBYE2ENM;
 189         dipcorrA[i] = 0;
 190         dipcorrB[i] = 0;
 191     }
 192     dipole_coeff = 0;
 193     switch (ewald_geometry)
 194     {
 195         case eewg3D:
 196             if (epsilon_surface != 0)
 197             {
 198                 dipole_coeff =
 199                     2*M_PI*ONE_4PI_EPS0/((2*epsilon_surface + fr->epsilon_r)*vol);
 200                 for (i = 0; (i < DIM); i++)
 201                 {
 202                     dipcorrA[i] = 2*dipole_coeff*mutot[0][i];
 203                     dipcorrB[i] = 2*dipole_coeff*mutot[1][i];
 204                 }
 205             }
 206             break;
 207         case eewg3DC:
 208             dipole_coeff = 2*M_PI*one_4pi_eps/vol;
 209             dipcorrA[ZZ] = 2*dipole_coeff*mutot[0][ZZ];
 210             dipcorrB[ZZ] = 2*dipole_coeff*mutot[1][ZZ];
 211             break;
 212         default:
 213             gmx_incons("Unsupported Ewald geometry");
 214             break;
 215     }
 216     if (debug)
 217     {
 218         fprintf(debug, "dipcorr = %8.3f  %8.3f  %8.3f\n",
 219                 dipcorrA[XX], dipcorrA[YY], dipcorrA[ZZ]);
 220         fprintf(debug, "mutot   = %8.3f  %8.3f  %8.3f\n",
 221                 mutot[0][XX], mutot[0][YY], mutot[0][ZZ]);
 222     }
 223     clear_mat(dxdf_q);
 224     if (EVDW_PME(fr->vdwtype))
 225     {
 226         clear_mat(dxdf_lj);
 227     }
 228     if ((calc_excl_corr || dipole_coeff != 0) && !bFreeEnergy)
 229     {
 230         for (i = start; (i < end); i++)
 231         {
 232             /* Initiate local variables (for this i-particle) to 0 */
 233             qiA = chargeA[i]*one_4pi_eps;
 234             if (EVDW_PME(fr->vdwtype))
 235             {
 236                 c6Ai = C6A[i];
 237                 if (bDoingLBRule)
 238                 {
 239                     c6Ai *= sigma3A[i];
 240                 }
 241             }
 242             if (calc_excl_corr)
 243             {
 244                 i1  = excl->index[i];
 245                 i2  = excl->index[i+1];
 246
 247                 /* Loop over excluded neighbours */
 248                 for (j = i1; (j < i2); j++)
 249                 {
 250                     k = AA[j];
 251                     /*
 252                      * First we must test whether k <> i, and then,
 253                      * because the exclusions are all listed twice i->k
 254                      * and k->i we must select just one of the two.  As
 255                      * a minor optimization we only compute forces when
 256                      * the charges are non-zero.
 257                      */
 258                     if (k > i)
 259                     {
 260                         qqA = qiA*chargeA[k];
 261                         if (EVDW_PME(fr->vdwtype))
 262                         {
 263                             c6A  = c6Ai * C6A[k];
 264                             if (bDoingLBRule)
 265                             {
 266                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 267                             }
 268                         }
 269                         if (qqA != 0.0 || c6A != 0.0)
 270                         {
 271                             real fscal;
 272
 273                             fscal = 0;
 274                             rvec_sub(x[i], x[k], dx);
 275                             if (bMolPBC)
 276                             {
 277                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 278                                 for (m = DIM-1; (m >= 0); m--)
 279                                 {
 280                                     if (dx[m] > 0.5*box[m][m])
 281                                     {
 282                                         rvec_dec(dx, box[m]);
 283                                     }
 284                                     else if (dx[m] < -0.5*box[m][m])
 285                                     {
 286                                         rvec_inc(dx, box[m]);
 287                                     }
 288                                 }
 289                             }
 290                             dr2 = norm2(dx);
 291                             /* Distance between two excluded particles
 292                              * may be zero in the case of shells
 293                              */
 294                             if (dr2 != 0)
 295                             {
 296                                 rinv              = gmx_invsqrt(dr2);
 297                                 rinv2             = rinv*rinv;
 298                                 if (qqA != 0.0)
 299                                 {
 300                                     real dr;
 301
 302                                     dr       = 1.0/rinv;
 303                                     ewcdr    = ewc_q*dr;
 304                                     vc       = qqA*gmx_erf(ewcdr)*rinv;
 305                                     Vexcl_q += vc;
 306 #ifdef GMX_DOUBLE
 307                                     /* Relative accuracy at R_ERF_R_INACC of 3e-10 */
 308 #define       R_ERF_R_INACC 0.006
 309 #else
 310                                     /* Relative accuracy at R_ERF_R_INACC of 2e-5 */
 311 #define       R_ERF_R_INACC 0.1
 312 #endif
 313                                     /* fscal is the scalar force pre-multiplied by rinv,
 314                                      * to normalise the relative position vector dx */
 315                                     if (ewcdr > R_ERF_R_INACC)
 316                                     {
 317                                         fscal = rinv2*(vc - qqA*ewc_q*M_2_SQRTPI*exp(-ewcdr*ewcdr));
 318                                     }
 319                                     else
 320                                     {
 321                                         /* Use a fourth order series expansion for small ewcdr */
 322                                         fscal = ewc_q*ewc_q*qqA*vr0_q*(2.0/3.0 - 0.4*ewcdr*ewcdr);
 323                                     }
 324
 325                                     /* The force vector is obtained by multiplication with
 326                                      * the relative position vector
 327                                      */
 328                                     svmul(fscal, dx, df);
 329                                     rvec_inc(f[k], df);
 330                                     rvec_dec(f[i], df);
 331                                     for (iv = 0; (iv < DIM); iv++)
 332                                     {
 333                                         for (jv = 0; (jv < DIM); jv++)
 334                                         {
 335                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 336                                         }
 337                                     }
 338                                 }
 339
 340                                 if (c6A != 0.0)
 341                                 {
 342                                     rinv6     = rinv2*rinv2*rinv2;
 343                                     ewcdr2    = ewc_lj2*dr2;
 344                                     ewcdr4    = ewcdr2*ewcdr2;
 345                                     /* We get the excluded long-range contribution from -C6*(1-g(r))
 346                                      * g(r) is also defined in the manual under LJ-PME
 347                                      */
 348                                     vc        = -c6A*rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 349                                     Vexcl_lj += vc;
 350                                     /* The force is the derivative of the potential vc.
 351                                      * fscal is the scalar force pre-multiplied by rinv,
 352                                      * to normalise the relative position vector dx */
 353                                     fscal     = 6.0*vc*rinv2 + c6A*rinv6*exp(-ewcdr2)*ewc_lj2*ewcdr4;
 354
 355                                     /* The force vector is obtained by multiplication with
 356                                      * the relative position vector
 357                                      */
 358                                     svmul(fscal, dx, df);
 359                                     rvec_inc(f[k], df);
 360                                     rvec_dec(f[i], df);
 361                                     for (iv = 0; (iv < DIM); iv++)
 362                                     {
 363                                         for (jv = 0; (jv < DIM); jv++)
 364                                         {
 365                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 366                                         }
 367                                     }
 368                                 }
 369                             }
 370                             else
 371                             {
 372                                 Vexcl_q  += qqA*vr0_q;
 373                                 Vexcl_lj += c6A*vr0_lj;
 374                             }
 375                         }
 376                     }
 377                 }
 378             }
 379             /* Dipole correction on force */
 380             if (dipole_coeff != 0)
 381             {
 382                 for (j = 0; (j < DIM); j++)
 383                 {
 384                     f[i][j] -= dipcorrA[j]*chargeA[i];
 385                 }
 386             }
 387         }
 388     }
 389     else if (calc_excl_corr || dipole_coeff != 0)
 390     {
 391         for (i = start; (i < end); i++)
 392         {
 393             /* Initiate local variables (for this i-particle) to 0 */
 394             qiA = chargeA[i]*one_4pi_eps;
 395             qiB = chargeB[i]*one_4pi_eps;
 396             if (EVDW_PME(fr->vdwtype))
 397             {
 398                 c6Ai = C6A[i];
 399                 c6Bi = C6B[i];
 400                 if (bDoingLBRule)
 401                 {
 402                     c6Ai *= sigma3A[i];
 403                     c6Bi *= sigma3B[i];
 404                 }
 405             }
 406             if (calc_excl_corr)
 407             {
 408                 i1  = excl->index[i];
 409                 i2  = excl->index[i+1];
 410
 411                 /* Loop over excluded neighbours */
 412                 for (j = i1; (j < i2); j++)
 413                 {
 414                     k = AA[j];
 415                     if (k > i)
 416                     {
 417                         qqA = qiA*chargeA[k];
 418                         qqB = qiB*chargeB[k];
 419                         if (EVDW_PME(fr->vdwtype))
 420                         {
 421                             c6A = c6Ai*C6A[k];
 422                             c6B = c6Bi*C6B[k];
 423                             if (bDoingLBRule)
 424                             {
 425                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 426                                 c6B *= pow(0.5*(sigmaB[i]+sigmaB[k]), 6)*sigma3B[k];
 427                             }
 428                         }
 429                         if (qqA != 0.0 || qqB != 0.0 || c6A != 0.0 || c6B != 0.0)
 430                         {
 431                             real fscal;
 432
 433                             fscal = 0;
 434                             qqL   = L1_q*qqA + lambda_q*qqB;
 435                             if (EVDW_PME(fr->vdwtype))
 436                             {
 437                                 c6L = L1_lj*c6A + lambda_lj*c6B;
 438                             }
 439                             rvec_sub(x[i], x[k], dx);
 440                             if (bMolPBC)
 441                             {
 442                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 443                                 for (m = DIM-1; (m >= 0); m--)
 444                                 {
 445                                     if (dx[m] > 0.5*box[m][m])
 446                                     {
 447                                         rvec_dec(dx, box[m]);
 448                                     }
 449                                     else if (dx[m] < -0.5*box[m][m])
 450                                     {
 451                                         rvec_inc(dx, box[m]);
 452                                     }
 453                                 }
 454                             }
 455                             dr2 = norm2(dx);
 456                             if (dr2 != 0)
 457                             {
 458                                 rinv    = gmx_invsqrt(dr2);
 459                                 rinv2   = rinv*rinv;
 460                                 if (qqA != 0.0 || qqB != 0.0)
 461                                 {
 462                                     real dr;
 463
 464                                     dr           = 1.0/rinv;
 465                                     v            = gmx_erf(ewc_q*dr)*rinv;
 466                                     vc           = qqL*v;
 467                                     Vexcl_q     += vc;
 468                                     /* fscal is the scalar force pre-multiplied by rinv,
 469                                      * to normalise the relative position vector dx */
 470                                     fscal        = rinv2*(vc-qqL*ewc_q*M_2_SQRTPI*exp(-ewc_q*ewc_q*dr2));
 471                                     dvdl_excl_q += (qqB - qqA)*v;
 472
 473                                     /* The force vector is obtained by multiplication with
 474                                      * the relative position vector
 475                                      */
 476                                     svmul(fscal, dx, df);
 477                                     rvec_inc(f[k], df);
 478                                     rvec_dec(f[i], df);
 479                                     for (iv = 0; (iv < DIM); iv++)
 480                                     {
 481                                         for (jv = 0; (jv < DIM); jv++)
 482                                         {
 483                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 484                                         }
 485                                     }
 486                                 }
 487
 488                                 if ((c6A != 0.0 || c6B != 0.0) && EVDW_PME(fr->vdwtype))
 489                                 {
 490                                     rinv6         = rinv2*rinv2*rinv2;
 491                                     ewcdr2        = ewc_lj2*dr2;
 492                                     ewcdr4        = ewcdr2*ewcdr2;
 493                                     v             = -rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 494                                     vc            = c6L*v;
 495                                     Vexcl_lj     += vc;
 496                                     /* fscal is the scalar force pre-multiplied by rinv,
 497                                      * to normalise the relative position vector dx */
 498                                     fscal         = 6.0*vc*rinv2 + c6L*rinv6*exp(-ewcdr2)*ewc_lj2*ewcdr4;
 499                                     dvdl_excl_lj += (c6B - c6A)*v;
 500
 501                                     /* The force vector is obtained by multiplication with
 502                                      * the relative position vector
 503                                      */
 504                                     svmul(fscal, dx, df);
 505                                     rvec_inc(f[k], df);
 506                                     rvec_dec(f[i], df);
 507                                     for (iv = 0; (iv < DIM); iv++)
 508                                     {
 509                                         for (jv = 0; (jv < DIM); jv++)
 510                                         {
 511                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 512                                         }
 513                                     }
 514                                 }
 515                             }
 516                             else
 517                             {
 518                                 Vexcl_q      += qqL*vr0_q;
 519                                 dvdl_excl_q  += (qqB - qqA)*vr0_q;
 520                                 Vexcl_lj     += c6L*vr0_lj;
 521                                 dvdl_excl_lj += (c6B - c6A)*vr0_lj;
 522                             }
 523                         }
 524                     }
 525                 }
 526             }
 527             /* Dipole correction on force */
 528             if (dipole_coeff != 0)
 529             {
 530                 for (j = 0; (j < DIM); j++)
 531                 {
 532                     f[i][j] -= L1_q*dipcorrA[j]*chargeA[i]
 533                         + lambda_q*dipcorrB[j]*chargeB[i];
 534                 }
 535             }
 536         }
 537     }
 538     for (iv = 0; (iv < DIM); iv++)
 539     {
 540         for (jv = 0; (jv < DIM); jv++)
 541         {
 542             vir_q[iv][jv]  += 0.5*dxdf_q[iv][jv];
 543             vir_lj[iv][jv] += 0.5*dxdf_lj[iv][jv];
 544         }
 545     }
 546
 547     Vself_q[0]  = 0;
 548     Vself_q[1]  = 0;
 549     Vself_lj[0] = 0;
 550     Vself_lj[1] = 0;
 551
 552     /* Global corrections only on master process */
 553     if (MASTER(cr) && thread == 0)
 554     {
 555         for (q = 0; q < (bFreeEnergy ? 2 : 1); q++)
 556         {
 557             if (calc_excl_corr)
 558             {
 559                 /* Self-energy correction */
 560                 Vself_q[q] = ewc_q*one_4pi_eps*fr->q2sum[q]*M_1_SQRTPI;
 561                 if (EVDW_PME(fr->vdwtype))
 562                 {
 563                     Vself_lj[q] =  fr->c6sum[q]*0.5*vr0_lj;
 564                 }
 565             }
 566
 567             /* Apply surface dipole correction:
 568              * correction = dipole_coeff * (dipole)^2
 569              */
 570             if (dipole_coeff != 0)
 571             {
 572                 if (ewald_geometry == eewg3D)
 573                 {
 574                     Vdipole[q] = dipole_coeff*iprod(mutot[q], mutot[q]);
 575                 }
 576                 else if (ewald_geometry == eewg3DC)
 577                 {
 578                     Vdipole[q] = dipole_coeff*mutot[q][ZZ]*mutot[q][ZZ];
 579                 }
 580             }
 581         }
 582     }
 583     if (!bFreeEnergy)
 584     {
 585         *Vcorr_q = Vdipole[0] - Vself_q[0] - Vexcl_q;
 586         if (EVDW_PME(fr->vdwtype))
 587         {
 588             *Vcorr_lj = -Vself_lj[0] - Vexcl_lj;
 589         }
 590     }
 591     else
 592     {
 593         *Vcorr_q = L1_q*(Vdipole[0] - Vself_q[0])
 594             + lambda_q*(Vdipole[1] - Vself_q[1])
 595             - Vexcl_q;
 596         *dvdlambda_q += Vdipole[1] - Vself_q[1]
 597             - (Vdipole[0] - Vself_q[0]) - dvdl_excl_q;
 598         if (EVDW_PME(fr->vdwtype))
 599         {
 600             *Vcorr_lj      = -(L1_lj*Vself_lj[0] + lambda_lj*Vself_lj[1]) - Vexcl_lj;
 601             *dvdlambda_lj += -Vself_lj[1] + Vself_lj[0] - dvdl_excl_lj;
 602         }
 603     }
 604
 605     if (debug)
 606     {
 607         fprintf(debug, "Long Range corrections for Ewald interactions:\n");
 608         fprintf(debug, "start=%d,natoms=%d\n", start, end-start);
 609         fprintf(debug, "q2sum = %g, Vself_q=%g c6sum = %g, Vself_lj=%g\n",
 610                 L1_q*fr->q2sum[0]+lambda_q*fr->q2sum[1], L1_q*Vself_q[0]+lambda_q*Vself_q[1], L1_lj*fr->c6sum[0]+lambda_lj*fr->c6sum[1], L1_lj*Vself_lj[0]+lambda_lj*Vself_lj[1]);
 611         fprintf(debug, "Electrostatic Long Range correction: Vexcl=%g\n", Vexcl_q);
 612         fprintf(debug, "Lennard-Jones Long Range correction: Vexcl=%g\n", Vexcl_lj);
 613         if (MASTER(cr) && thread == 0)
 614         {
 615             if (epsilon_surface > 0 || ewald_geometry == eewg3DC)
 616             {
 617                 fprintf(debug, "Total dipole correction: Vdipole=%g\n",
 618                         L1_q*Vdipole[0]+lambda_q*Vdipole[1]);
 619             }
 620         }
 621     }
 622 }
 623
 624 real ewald_charge_correction(t_commrec *cr, t_forcerec *fr, real lambda,
 625                              matrix box,
 626                              real *dvdlambda, tensor vir)
 627
 628 {
 629     real vol, fac, qs2A, qs2B, vc, enercorr;
 630     int  d;
 631
 632     if (MASTER(cr))
 633     {
 634         /* Apply charge correction */
 635         vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 636
 637         fac = M_PI*ONE_4PI_EPS0/(fr->epsilon_r*2.0*vol*vol*sqr(fr->ewaldcoeff_q));
 638
 639         qs2A = fr->qsum[0]*fr->qsum[0];
 640         qs2B = fr->qsum[1]*fr->qsum[1];
 641
 642         vc = (qs2A*(1 - lambda) + qs2B*lambda)*fac;
 643
 644         enercorr = -vol*vc;
 645
 646         *dvdlambda += -vol*(qs2B - qs2A)*fac;
 647
 648         for (d = 0; d < DIM; d++)
 649         {
 650             vir[d][d] += vc;
 651         }
 652
 653         if (debug)
 654         {
 655             fprintf(debug, "Total charge correction: Vcharge=%g\n", enercorr);
 656         }
 657     }
 658     else
 659     {
 660         enercorr = 0;
 661     }
 662
 663     return enercorr;
 664 }