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