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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  12  * modify it under the terms of the GNU Lesser General Public License
  13  * as published by the Free Software Foundation; either version 2.1
  14  * of the License, or (at your option) any later version.
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  19  * Lesser General Public License for more details.
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  37 #ifdef HAVE_CONFIG_H
  38 #include <config.h>
  39 #endif
  40
  41 #include <stdio.h>
  42 #include <math.h>
  43 #include "gromacs/math/utilities.h"
  44 #include "typedefs.h"
  45 #include "vec.h"
  46 #include "coulomb.h"
  47 #include "smalloc.h"
  48 #include "physics.h"
  49 #include "txtdump.h"
  50 #include "gromacs/fileio/futil.h"
  51 #include "names.h"
  52 #include "macros.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, dr, dr2, rinv, fscal, 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;
 153     real        c6Ai   = 0, c6Bi = 0, c6A = 0, c6B = 0, ewcdr2, ewcdr4, ewcdr5, 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 || EVDW_PME(fr->vdwtype)) && !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                             rvec_sub(x[i], x[k], dx);
 272                             fscal = 0;
 273                             if (bMolPBC)
 274                             {
 275                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 276                                 for (m = DIM-1; (m >= 0); m--)
 277                                 {
 278                                     if (dx[m] > 0.5*box[m][m])
 279                                     {
 280                                         rvec_dec(dx, box[m]);
 281                                     }
 282                                     else if (dx[m] < -0.5*box[m][m])
 283                                     {
 284                                         rvec_inc(dx, box[m]);
 285                                     }
 286                                 }
 287                             }
 288                             dr2 = norm2(dx);
 289                             /* Distance between two excluded particles
 290                              * may be zero in the case of shells
 291                              */
 292                             if (dr2 != 0)
 293                             {
 294                                 rinv              = gmx_invsqrt(dr2);
 295                                 rinv2             = rinv*rinv;
 296                                 dr                = 1.0/rinv;
 297                                 if (qqA != 0.0)
 298                                 {
 299                                     ewcdr    = ewc_q*dr;
 300                                     vc       = qqA*gmx_erf(ewcdr)*rinv;
 301                                     Vexcl_q += vc;
 302 #ifdef GMX_DOUBLE
 303                                     /* Relative accuracy at R_ERF_R_INACC of 3e-10 */
 304 #define       R_ERF_R_INACC 0.006
 305 #else
 306                                     /* Relative accuracy at R_ERF_R_INACC of 2e-5 */
 307 #define       R_ERF_R_INACC 0.1
 308 #endif
 309                                     if (ewcdr > R_ERF_R_INACC)
 310                                     {
 311                                         fscal = rinv2*(vc - qqA*ewc_q*M_2_SQRTPI*exp(-ewcdr*ewcdr));
 312                                     }
 313                                     else
 314                                     {
 315                                         /* Use a fourth order series expansion for small ewcdr */
 316                                         fscal = ewc_q*ewc_q*qqA*vr0_q*(2.0/3.0 - 0.4*ewcdr*ewcdr);
 317                                     }
 318
 319                                     /* The force vector is obtained by multiplication with
 320                                      * the distance vector
 321                                      */
 322                                     svmul(fscal, dx, df);
 323                                     rvec_inc(f[k], df);
 324                                     rvec_dec(f[i], df);
 325                                     for (iv = 0; (iv < DIM); iv++)
 326                                     {
 327                                         for (jv = 0; (jv < DIM); jv++)
 328                                         {
 329                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 330                                         }
 331                                     }
 332                                 }
 333
 334                                 if (c6A != 0.0)
 335                                 {
 336                                     rinv6     = rinv2*rinv2*rinv2;
 337                                     ewcdr     = ewc_lj*dr;
 338                                     ewcdr2    = ewcdr*ewcdr;
 339                                     ewcdr4    = ewcdr2*ewcdr2;
 340                                     ewcdr5    = ewcdr4*ewcdr;
 341                                     /* We get the excluded long-range contribution from -C6*(1-g(r))
 342                                      * g(r) is also defined in the manual under LJ-PME
 343                                      */
 344                                     vc        = -c6A*rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 345                                     Vexcl_lj += vc;
 346                                     /* The force is the derivative of the potential vc */
 347                                     fscal     = 6.0*vc*rinv + c6A*rinv6*exp(-ewcdr2)*ewc_lj*ewcdr5;
 348
 349                                     /* The force vector is obtained by multiplication with
 350                                      * the distance vector. Here we also divide by r, to account
 351                                      * for the length of dx
 352                                      */
 353                                     svmul(fscal*rinv, dx, df);
 354                                     rvec_inc(f[k], df);
 355                                     rvec_dec(f[i], df);
 356                                     for (iv = 0; (iv < DIM); iv++)
 357                                     {
 358                                         for (jv = 0; (jv < DIM); jv++)
 359                                         {
 360                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 361                                         }
 362                                     }
 363                                 }
 364                             }
 365                             else
 366                             {
 367                                 Vexcl_q  += qqA*vr0_q;
 368                                 Vexcl_lj += c6A*vr0_lj;
 369                             }
 370                         }
 371                     }
 372                 }
 373             }
 374             /* Dipole correction on force */
 375             if (dipole_coeff != 0)
 376             {
 377                 for (j = 0; (j < DIM); j++)
 378                 {
 379                     f[i][j] -= dipcorrA[j]*chargeA[i];
 380                 }
 381             }
 382         }
 383     }
 384     else if (calc_excl_corr || dipole_coeff != 0 || EVDW_PME(fr->vdwtype))
 385     {
 386         for (i = start; (i < end); i++)
 387         {
 388             /* Initiate local variables (for this i-particle) to 0 */
 389             qiA = chargeA[i]*one_4pi_eps;
 390             qiB = chargeB[i]*one_4pi_eps;
 391             if (EVDW_PME(fr->vdwtype))
 392             {
 393                 c6Ai = C6A[i];
 394                 c6Bi = C6B[i];
 395                 if (bDoingLBRule)
 396                 {
 397                     c6Ai *= sigma3A[i];
 398                     c6Bi *= sigma3B[i];
 399                 }
 400             }
 401             if (calc_excl_corr)
 402             {
 403                 i1  = excl->index[i];
 404                 i2  = excl->index[i+1];
 405
 406                 /* Loop over excluded neighbours */
 407                 for (j = i1; (j < i2); j++)
 408                 {
 409                     k = AA[j];
 410                     if (k > i)
 411                     {
 412                         qqA = qiA*chargeA[k];
 413                         qqB = qiB*chargeB[k];
 414                         if (EVDW_PME(fr->vdwtype))
 415                         {
 416                             c6A = c6Ai*C6A[k];
 417                             c6B = c6Bi*C6B[k];
 418                             if (bDoingLBRule)
 419                             {
 420                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 421                                 c6B *= pow(0.5*(sigmaB[i]+sigmaB[k]), 6)*sigma3B[k];
 422                             }
 423                         }
 424                         if (qqA != 0.0 || qqB != 0.0 || c6A != 0.0 || c6B != 0.0)
 425                         {
 426                             fscal = 0;
 427                             qqL   = L1_q*qqA + lambda_q*qqB;
 428                             if (EVDW_PME(fr->vdwtype))
 429                             {
 430                                 c6L = L1_lj*c6A + lambda_lj*c6B;
 431                             }
 432                             rvec_sub(x[i], x[k], dx);
 433                             if (bMolPBC)
 434                             {
 435                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 436                                 for (m = DIM-1; (m >= 0); m--)
 437                                 {
 438                                     if (dx[m] > 0.5*box[m][m])
 439                                     {
 440                                         rvec_dec(dx, box[m]);
 441                                     }
 442                                     else if (dx[m] < -0.5*box[m][m])
 443                                     {
 444                                         rvec_inc(dx, box[m]);
 445                                     }
 446                                 }
 447                             }
 448                             dr2 = norm2(dx);
 449                             if (dr2 != 0)
 450                             {
 451                                 rinv    = gmx_invsqrt(dr2);
 452                                 rinv2   = rinv*rinv;
 453                                 dr      = 1.0/rinv;
 454                                 if (qqA != 0.0 || qqB != 0.0)
 455                                 {
 456                                     v            = gmx_erf(ewc_q*dr)*rinv;
 457                                     vc           = qqL*v;
 458                                     Vexcl_q     += vc;
 459                                     fscal        = rinv2*(vc-qqL*ewc_q*M_2_SQRTPI*exp(-ewc_q*ewc_q*dr2));
 460                                     dvdl_excl_q += (qqB - qqA)*v;
 461
 462                                     svmul(fscal, dx, df);
 463                                     rvec_inc(f[k], df);
 464                                     rvec_dec(f[i], df);
 465                                     for (iv = 0; (iv < DIM); iv++)
 466                                     {
 467                                         for (jv = 0; (jv < DIM); jv++)
 468                                         {
 469                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 470                                         }
 471                                     }
 472                                 }
 473
 474                                 if ((c6A != 0.0 || c6B != 0.0) && EVDW_PME(fr->vdwtype))
 475                                 {
 476                                     rinv6         = rinv2*rinv2*rinv2;
 477                                     ewcdr         = ewc_lj*dr;
 478                                     ewcdr2        = ewcdr*ewcdr;
 479                                     ewcdr4        = ewcdr2*ewcdr2;
 480                                     ewcdr5        = ewcdr4*ewcdr;
 481                                     v             = -rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 482                                     vc            = c6L*v;
 483                                     Vexcl_lj     += vc;
 484                                     fscal         = 6.0*vc*rinv + c6L*rinv6*exp(-ewcdr2)*ewc_lj*ewcdr5;
 485                                     dvdl_excl_lj += (c6B - c6A)*v;
 486
 487                                     /* The force vector is obtained by multiplication with the
 488                                      * distance vector. Here we also divide by r, to account
 489                                      * for the length of dx
 490                                      */
 491                                     svmul(fscal*rinv, dx, df);
 492                                     rvec_inc(f[k], df);
 493                                     rvec_dec(f[i], df);
 494                                     for (iv = 0; (iv < DIM); iv++)
 495                                     {
 496                                         for (jv = 0; (jv < DIM); jv++)
 497                                         {
 498                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 499                                         }
 500                                     }
 501                                 }
 502                             }
 503                             else
 504                             {
 505                                 Vexcl_q      += qqL*vr0_q;
 506                                 dvdl_excl_q  += (qqB - qqA)*vr0_q;
 507                                 Vexcl_lj     += c6L*vr0_lj;
 508                                 dvdl_excl_lj += (c6B - c6A)*vr0_lj;
 509                             }
 510                         }
 511                     }
 512                 }
 513             }
 514             /* Dipole correction on force */
 515             if (dipole_coeff != 0)
 516             {
 517                 for (j = 0; (j < DIM); j++)
 518                 {
 519                     f[i][j] -= L1_q*dipcorrA[j]*chargeA[i]
 520                         + lambda_q*dipcorrB[j]*chargeB[i];
 521                 }
 522             }
 523         }
 524     }
 525     for (iv = 0; (iv < DIM); iv++)
 526     {
 527         for (jv = 0; (jv < DIM); jv++)
 528         {
 529             vir_q[iv][jv]  += 0.5*dxdf_q[iv][jv];
 530             vir_lj[iv][jv] += 0.5*dxdf_lj[iv][jv];
 531         }
 532     }
 533
 534     Vself_q[0]  = 0;
 535     Vself_q[1]  = 0;
 536     Vself_lj[0] = 0;
 537     Vself_lj[1] = 0;
 538
 539     /* Global corrections only on master process */
 540     if (MASTER(cr) && thread == 0)
 541     {
 542         for (q = 0; q < (bFreeEnergy ? 2 : 1); q++)
 543         {
 544             if (calc_excl_corr)
 545             {
 546                 /* Self-energy correction */
 547                 Vself_q[q] = ewc_q*one_4pi_eps*fr->q2sum[q]*M_1_SQRTPI;
 548                 if (EVDW_PME(fr->vdwtype))
 549                 {
 550                     Vself_lj[q] = -pow(ewc_lj, 6)*fr->c6sum[q];
 551                 }
 552             }
 553
 554             /* Apply surface dipole correction:
 555              * correction = dipole_coeff * (dipole)^2
 556              */
 557             if (dipole_coeff != 0)
 558             {
 559                 if (ewald_geometry == eewg3D)
 560                 {
 561                     Vdipole[q] = dipole_coeff*iprod(mutot[q], mutot[q]);
 562                 }
 563                 else if (ewald_geometry == eewg3DC)
 564                 {
 565                     Vdipole[q] = dipole_coeff*mutot[q][ZZ]*mutot[q][ZZ];
 566                 }
 567             }
 568         }
 569     }
 570     if (!bFreeEnergy)
 571     {
 572         *Vcorr_q = Vdipole[0] - Vself_q[0] - Vexcl_q;
 573         if (EVDW_PME(fr->vdwtype))
 574         {
 575             *Vcorr_lj = -Vself_lj[0] - Vexcl_lj;
 576         }
 577     }
 578     else
 579     {
 580         *Vcorr_q = L1_q*(Vdipole[0] - Vself_q[0])
 581             + lambda_q*(Vdipole[1] - Vself_q[1])
 582             - Vexcl_q;
 583         *dvdlambda_q += Vdipole[1] - Vself_q[1]
 584             - (Vdipole[0] - Vself_q[0]) - dvdl_excl_q;
 585         if (EVDW_PME(fr->vdwtype))
 586         {
 587             *Vcorr_lj      = -(L1_lj*Vself_lj[0] + lambda_lj*Vself_lj[1]) - Vexcl_lj;
 588             *dvdlambda_lj += -Vself_lj[1] + Vself_lj[0] - dvdl_excl_lj;
 589         }
 590     }
 591
 592     if (debug)
 593     {
 594         fprintf(debug, "Long Range corrections for Ewald interactions:\n");
 595         fprintf(debug, "start=%d,natoms=%d\n", start, end-start);
 596         fprintf(debug, "q2sum = %g, Vself_q=%g c6sum = %g, Vself_lj=%g\n",
 597                 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]);
 598         fprintf(debug, "Electrostatic Long Range correction: Vexcl=%g\n", Vexcl_q);
 599         fprintf(debug, "Lennard-Jones Long Range correction: Vexcl=%g\n", Vexcl_lj);
 600         if (MASTER(cr) && thread == 0)
 601         {
 602             if (epsilon_surface > 0 || ewald_geometry == eewg3DC)
 603             {
 604                 fprintf(debug, "Total dipole correction: Vdipole=%g\n",
 605                         L1_q*Vdipole[0]+lambda_q*Vdipole[1]);
 606             }
 607         }
 608     }
 609 }
 610
 611 real ewald_charge_correction(t_commrec *cr, t_forcerec *fr, real lambda,
 612                              matrix box,
 613                              real *dvdlambda, tensor vir)
 614
 615 {
 616     real vol, fac, qs2A, qs2B, vc, enercorr;
 617     int  d;
 618
 619     if (MASTER(cr))
 620     {
 621         /* Apply charge correction */
 622         vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 623
 624         fac = M_PI*ONE_4PI_EPS0/(fr->epsilon_r*2.0*vol*vol*sqr(fr->ewaldcoeff_q));
 625
 626         qs2A = fr->qsum[0]*fr->qsum[0];
 627         qs2B = fr->qsum[1]*fr->qsum[1];
 628
 629         vc = (qs2A*(1 - lambda) + qs2B*lambda)*fac;
 630
 631         enercorr = -vol*vc;
 632
 633         *dvdlambda += -vol*(qs2B - qs2A)*fac;
 634
 635         for (d = 0; d < DIM; d++)
 636         {
 637             vir[d][d] += vc;
 638         }
 639
 640         if (debug)
 641         {
 642             fprintf(debug, "Total charge correction: Vcharge=%g\n", enercorr);
 643         }
 644     }
 645     else
 646     {
 647         enercorr = 0;
 648     }
 649
 650     return enercorr;
 651 }