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 "writeps.h"
  53 #include "macros.h"
  54
  55 real calc_ewaldcoeff_q(real rc, real dtol)
  56 {
  57     real x = 5, low, high;
  58     int  n, i = 0;
  59
  60
  61     do
  62     {
  63         i++;
  64         x *= 2;
  65     }
  66     while (gmx_erfc(x*rc) > dtol);
  67
  68     n    = i+60; /* search tolerance is 2^-60 */
  69     low  = 0;
  70     high = x;
  71     for (i = 0; i < n; i++)
  72     {
  73         x = (low+high)/2;
  74         if (gmx_erfc(x*rc) > dtol)
  75         {
  76             low = x;
  77         }
  78         else
  79         {
  80             high = x;
  81         }
  82     }
  83     return x;
  84 }
  85
  86 static real ewald_function_lj(real x, real rc)
  87 {
  88     real xrc, xrc2, xrc4, factor;
  89     xrc  = x*rc;
  90     xrc2 = xrc*xrc;
  91     xrc4 = xrc2*xrc2;
  92 #ifdef GMX_DOUBLE
  93     factor = exp(-xrc2)*(1 + xrc2 + xrc4/2.0);
  94 #else
  95     factor = expf(-xrc2)*(1 + xrc2 + xrc4/2.0);
  96 #endif
  97
  98     return factor;
  99 }
 100
 101 real calc_ewaldcoeff_lj(real rc, real dtol)
 102 {
 103     real x = 5, low, high;
 104     int  n, i = 0;
 105
 106     do
 107     {
 108         i++;
 109         x *= 2.0;
 110     }
 111     while (ewald_function_lj(x, rc) > dtol);
 112
 113     n    = i + 60; /* search tolerance is 2^-60 */
 114     low  = 0;
 115     high = x;
 116     for (i = 0; i < n; ++i)
 117     {
 118         x = (low + high) / 2.0;
 119         if (ewald_function_lj(x, rc) > dtol)
 120         {
 121             low = x;
 122         }
 123         else
 124         {
 125             high = x;
 126         }
 127     }
 128     return x;
 129 }
 130
 131 void ewald_LRcorrection(int start, int end,
 132                         t_commrec *cr, int thread, t_forcerec *fr,
 133                         real *chargeA, real *chargeB,
 134                         real *C6A, real *C6B,
 135                         real *sigmaA, real *sigmaB,
 136                         real *sigma3A, real *sigma3B,
 137                         gmx_bool calc_excl_corr,
 138                         t_blocka *excl, rvec x[],
 139                         matrix box, rvec mu_tot[],
 140                         int ewald_geometry, real epsilon_surface,
 141                         rvec *f, tensor vir_q, tensor vir_lj,
 142                         real *Vcorr_q, real *Vcorr_lj,
 143                         real lambda_q, real lambda_lj,
 144                         real *dvdlambda_q, real *dvdlambda_lj)
 145 {
 146     int         i, i1, i2, j, k, m, iv, jv, q;
 147     atom_id    *AA;
 148     double      Vexcl_q, dvdl_excl_q, dvdl_excl_lj; /* Necessary for precision */
 149     double      Vexcl_lj;
 150     real        one_4pi_eps;
 151     real        v, vc, qiA, qiB, dr, dr2, rinv, fscal, enercorr;
 152     real        Vself_q[2], Vself_lj[2], Vdipole[2], rinv2, ewc_q = fr->ewaldcoeff_q, ewcdr;
 153     real        ewc_lj = fr->ewaldcoeff_lj;
 154     real        c6Ai   = 0, c6Bi = 0, c6A = 0, c6B = 0, ewcdr2, ewcdr4, ewcdr5, c6L = 0, rinv6;
 155     rvec        df, dx, mutot[2], dipcorrA, dipcorrB;
 156     tensor      dxdf_q, dxdf_lj;
 157     real        vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 158     real        L1_q, L1_lj, dipole_coeff, qqA, qqB, qqL, vr0_q, vr0_lj = 0;
 159     gmx_bool    bFreeEnergy  = (chargeB != NULL);
 160     gmx_bool    bMolPBC      = fr->bMolPBC;
 161     gmx_bool    bDoingLBRule = (fr->ljpme_combination_rule == eljpmeLB);
 162
 163     /* This routine can be made faster by using tables instead of analytical interactions
 164      * However, that requires a thorough verification that they are correct in all cases.
 165      */
 166
 167     one_4pi_eps   = ONE_4PI_EPS0/fr->epsilon_r;
 168     vr0_q         = ewc_q*M_2_SQRTPI;
 169     if (EVDW_PME(fr->vdwtype))
 170     {
 171         vr0_lj        = -pow(ewc_lj, 6)/6.0;
 172     }
 173
 174     AA           = excl->a;
 175     Vexcl_q      = 0;
 176     Vexcl_lj     = 0;
 177     dvdl_excl_q  = 0;
 178     dvdl_excl_lj = 0;
 179     Vdipole[0]   = 0;
 180     Vdipole[1]   = 0;
 181     L1_q         = 1.0-lambda_q;
 182     L1_lj        = 1.0-lambda_lj;
 183     /* Note that we have to transform back to gromacs units, since
 184      * mu_tot contains the dipole in debye units (for output).
 185      */
 186     for (i = 0; (i < DIM); i++)
 187     {
 188         mutot[0][i] = mu_tot[0][i]*DEBYE2ENM;
 189         mutot[1][i] = mu_tot[1][i]*DEBYE2ENM;
 190         dipcorrA[i] = 0;
 191         dipcorrB[i] = 0;
 192     }
 193     dipole_coeff = 0;
 194     switch (ewald_geometry)
 195     {
 196         case eewg3D:
 197             if (epsilon_surface != 0)
 198             {
 199                 dipole_coeff =
 200                     2*M_PI*ONE_4PI_EPS0/((2*epsilon_surface + fr->epsilon_r)*vol);
 201                 for (i = 0; (i < DIM); i++)
 202                 {
 203                     dipcorrA[i] = 2*dipole_coeff*mutot[0][i];
 204                     dipcorrB[i] = 2*dipole_coeff*mutot[1][i];
 205                 }
 206             }
 207             break;
 208         case eewg3DC:
 209             dipole_coeff = 2*M_PI*one_4pi_eps/vol;
 210             dipcorrA[ZZ] = 2*dipole_coeff*mutot[0][ZZ];
 211             dipcorrB[ZZ] = 2*dipole_coeff*mutot[1][ZZ];
 212             break;
 213         default:
 214             gmx_incons("Unsupported Ewald geometry");
 215             break;
 216     }
 217     if (debug)
 218     {
 219         fprintf(debug, "dipcorr = %8.3f  %8.3f  %8.3f\n",
 220                 dipcorrA[XX], dipcorrA[YY], dipcorrA[ZZ]);
 221         fprintf(debug, "mutot   = %8.3f  %8.3f  %8.3f\n",
 222                 mutot[0][XX], mutot[0][YY], mutot[0][ZZ]);
 223     }
 224     clear_mat(dxdf_q);
 225     if (EVDW_PME(fr->vdwtype))
 226     {
 227         clear_mat(dxdf_lj);
 228     }
 229     if ((calc_excl_corr || dipole_coeff != 0 || EVDW_PME(fr->vdwtype)) && !bFreeEnergy)
 230     {
 231         for (i = start; (i < end); i++)
 232         {
 233             /* Initiate local variables (for this i-particle) to 0 */
 234             qiA = chargeA[i]*one_4pi_eps;
 235             if (EVDW_PME(fr->vdwtype))
 236             {
 237                 c6Ai = C6A[i];
 238                 if (bDoingLBRule)
 239                 {
 240                     c6Ai *= sigma3A[i];
 241                 }
 242             }
 243             if (calc_excl_corr)
 244             {
 245                 i1  = excl->index[i];
 246                 i2  = excl->index[i+1];
 247
 248                 /* Loop over excluded neighbours */
 249                 for (j = i1; (j < i2); j++)
 250                 {
 251                     k = AA[j];
 252                     /*
 253                      * First we must test whether k <> i, and then,
 254                      * because the exclusions are all listed twice i->k
 255                      * and k->i we must select just one of the two.  As
 256                      * a minor optimization we only compute forces when
 257                      * the charges are non-zero.
 258                      */
 259                     if (k > i)
 260                     {
 261                         qqA = qiA*chargeA[k];
 262                         if (EVDW_PME(fr->vdwtype))
 263                         {
 264                             c6A  = c6Ai * C6A[k];
 265                             if (bDoingLBRule)
 266                             {
 267                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 268                             }
 269                         }
 270                         if (qqA != 0.0 || c6A != 0.0)
 271                         {
 272                             rvec_sub(x[i], x[k], dx);
 273                             fscal = 0;
 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                                 dr                = 1.0/rinv;
 298                                 if (qqA != 0.0)
 299                                 {
 300                                     ewcdr    = ewc_q*dr;
 301                                     vc       = qqA*gmx_erf(ewcdr)*rinv;
 302                                     Vexcl_q += vc;
 303 #ifdef GMX_DOUBLE
 304                                     /* Relative accuracy at R_ERF_R_INACC of 3e-10 */
 305 #define       R_ERF_R_INACC 0.006
 306 #else
 307                                     /* Relative accuracy at R_ERF_R_INACC of 2e-5 */
 308 #define       R_ERF_R_INACC 0.1
 309 #endif
 310                                     if (ewcdr > R_ERF_R_INACC)
 311                                     {
 312                                         fscal = rinv2*(vc - qqA*ewc_q*M_2_SQRTPI*exp(-ewcdr*ewcdr));
 313                                     }
 314                                     else
 315                                     {
 316                                         /* Use a fourth order series expansion for small ewcdr */
 317                                         fscal = ewc_q*ewc_q*qqA*vr0_q*(2.0/3.0 - 0.4*ewcdr*ewcdr);
 318                                     }
 319
 320                                     /* The force vector is obtained by multiplication with
 321                                      * the distance vector
 322                                      */
 323                                     svmul(fscal, dx, df);
 324                                     rvec_inc(f[k], df);
 325                                     rvec_dec(f[i], df);
 326                                     for (iv = 0; (iv < DIM); iv++)
 327                                     {
 328                                         for (jv = 0; (jv < DIM); jv++)
 329                                         {
 330                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 331                                         }
 332                                     }
 333                                 }
 334
 335                                 if (c6A != 0.0)
 336                                 {
 337                                     rinv6     = rinv2*rinv2*rinv2;
 338                                     ewcdr     = ewc_lj*dr;
 339                                     ewcdr2    = ewcdr*ewcdr;
 340                                     ewcdr4    = ewcdr2*ewcdr2;
 341                                     ewcdr5    = ewcdr4*ewcdr;
 342                                     /* We get the excluded long-range contribution from -C6*(1-g(r))
 343                                      * g(r) is also defined in the manual under LJ-PME
 344                                      */
 345                                     vc        = -c6A*rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 346                                     Vexcl_lj += vc;
 347                                     /* The force is the derivative of the potential vc */
 348                                     fscal     = 6.0*vc*rinv + c6A*rinv6*exp(-ewcdr2)*ewc_lj*ewcdr5;
 349
 350                                     /* The force vector is obtained by
 351                                      * multiplication with the distance
 352                                      * vector
 353                                      */
 354                                     svmul(fscal, dx, df);
 355                                     rvec_inc(f[k], df);
 356                                     rvec_dec(f[i], df);
 357                                     for (iv = 0; (iv < DIM); iv++)
 358                                     {
 359                                         for (jv = 0; (jv < DIM); jv++)
 360                                         {
 361                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 362                                         }
 363                                     }
 364                                 }
 365                             }
 366                             else
 367                             {
 368                                 Vexcl_q  += qqA*vr0_q;
 369                                 Vexcl_lj += c6A*vr0_lj;
 370                             }
 371                         }
 372                     }
 373                 }
 374             }
 375             /* Dipole correction on force */
 376             if (dipole_coeff != 0)
 377             {
 378                 for (j = 0; (j < DIM); j++)
 379                 {
 380                     f[i][j] -= dipcorrA[j]*chargeA[i];
 381                 }
 382             }
 383         }
 384     }
 385     else if (calc_excl_corr || dipole_coeff != 0 || EVDW_PME(fr->vdwtype))
 386     {
 387         for (i = start; (i < end); i++)
 388         {
 389             /* Initiate local variables (for this i-particle) to 0 */
 390             qiA = chargeA[i]*one_4pi_eps;
 391             qiB = chargeB[i]*one_4pi_eps;
 392             if (EVDW_PME(fr->vdwtype))
 393             {
 394                 c6Ai = C6A[i];
 395                 c6Bi = C6B[i];
 396                 if (bDoingLBRule)
 397                 {
 398                     c6Ai *= sigma3A[i];
 399                     c6Bi *= sigma3B[i];
 400                 }
 401             }
 402             if (calc_excl_corr)
 403             {
 404                 i1  = excl->index[i];
 405                 i2  = excl->index[i+1];
 406
 407                 /* Loop over excluded neighbours */
 408                 for (j = i1; (j < i2); j++)
 409                 {
 410                     k = AA[j];
 411                     if (k > i)
 412                     {
 413                         qqA = qiA*chargeA[k];
 414                         qqB = qiB*chargeB[k];
 415                         if (EVDW_PME(fr->vdwtype))
 416                         {
 417                             c6A = c6Ai*C6A[k];
 418                             c6B = c6Bi*C6B[k];
 419                             if (bDoingLBRule)
 420                             {
 421                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 422                                 c6B *= pow(0.5*(sigmaB[i]+sigmaB[k]), 6)*sigma3B[k];
 423                             }
 424                         }
 425                         if (qqA != 0.0 || qqB != 0.0 || c6A != 0.0 || c6B != 0.0)
 426                         {
 427                             fscal = 0;
 428                             qqL   = L1_q*qqA + lambda_q*qqB;
 429                             if (EVDW_PME(fr->vdwtype))
 430                             {
 431                                 c6L = L1_lj*c6A + lambda_lj*c6B;
 432                             }
 433                             rvec_sub(x[i], x[k], dx);
 434                             if (bMolPBC)
 435                             {
 436                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 437                                 for (m = DIM-1; (m >= 0); m--)
 438                                 {
 439                                     if (dx[m] > 0.5*box[m][m])
 440                                     {
 441                                         rvec_dec(dx, box[m]);
 442                                     }
 443                                     else if (dx[m] < -0.5*box[m][m])
 444                                     {
 445                                         rvec_inc(dx, box[m]);
 446                                     }
 447                                 }
 448                             }
 449                             dr2 = norm2(dx);
 450                             if (dr2 != 0)
 451                             {
 452                                 rinv    = gmx_invsqrt(dr2);
 453                                 rinv2   = rinv*rinv;
 454                                 dr      = 1.0/rinv;
 455                                 if (qqA != 0.0 || qqB != 0.0)
 456                                 {
 457                                     v            = gmx_erf(ewc_q*dr)*rinv;
 458                                     vc           = qqL*v;
 459                                     Vexcl_q     += vc;
 460                                     fscal        = rinv2*(vc-qqL*ewc_q*M_2_SQRTPI*exp(-ewc_q*ewc_q*dr2));
 461                                     dvdl_excl_q += (qqB - qqA)*v;
 462
 463                                     svmul(fscal, dx, df);
 464                                     rvec_inc(f[k], df);
 465                                     rvec_dec(f[i], df);
 466                                     for (iv = 0; (iv < DIM); iv++)
 467                                     {
 468                                         for (jv = 0; (jv < DIM); jv++)
 469                                         {
 470                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 471                                         }
 472                                     }
 473                                 }
 474
 475                                 if ((c6A != 0.0 || c6B != 0.0) && EVDW_PME(fr->vdwtype))
 476                                 {
 477                                     rinv6         = rinv2*rinv2*rinv2;
 478                                     ewcdr         = ewc_lj*dr;
 479                                     ewcdr2        = ewcdr*ewcdr;
 480                                     ewcdr4        = ewcdr2*ewcdr2;
 481                                     ewcdr5        = ewcdr4*ewcdr;
 482                                     v             = -rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 483                                     vc            = c6L*v;
 484                                     Vexcl_lj     += vc;
 485                                     fscal         = 6.0*vc*rinv + c6L*rinv6*exp(-ewcdr2)*ewc_lj*ewcdr5;
 486                                     dvdl_excl_lj += (c6B - c6A)*v;
 487                                     svmul(fscal, dx, df);
 488                                     rvec_inc(f[k], df);
 489                                     rvec_dec(f[i], df);
 490                                     for (iv = 0; (iv < DIM); iv++)
 491                                     {
 492                                         for (jv = 0; (jv < DIM); jv++)
 493                                         {
 494                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 495                                         }
 496                                     }
 497                                 }
 498                             }
 499                             else
 500                             {
 501                                 Vexcl_q      += qqL*vr0_q;
 502                                 dvdl_excl_q  += (qqB - qqA)*vr0_q;
 503                                 Vexcl_lj     += c6L*vr0_lj;
 504                                 dvdl_excl_lj += (c6B - c6A)*vr0_lj;
 505                             }
 506                         }
 507                     }
 508                 }
 509             }
 510             /* Dipole correction on force */
 511             if (dipole_coeff != 0)
 512             {
 513                 for (j = 0; (j < DIM); j++)
 514                 {
 515                     f[i][j] -= L1_q*dipcorrA[j]*chargeA[i]
 516                         + lambda_q*dipcorrB[j]*chargeB[i];
 517                 }
 518             }
 519         }
 520     }
 521     for (iv = 0; (iv < DIM); iv++)
 522     {
 523         for (jv = 0; (jv < DIM); jv++)
 524         {
 525             vir_q[iv][jv]  += 0.5*dxdf_q[iv][jv];
 526             vir_lj[iv][jv] += 0.5*dxdf_lj[iv][jv];
 527         }
 528     }
 529
 530     Vself_q[0]  = 0;
 531     Vself_q[1]  = 0;
 532     Vself_lj[0] = 0;
 533     Vself_lj[1] = 0;
 534
 535     /* Global corrections only on master process */
 536     if (MASTER(cr) && thread == 0)
 537     {
 538         for (q = 0; q < (bFreeEnergy ? 2 : 1); q++)
 539         {
 540             if (calc_excl_corr)
 541             {
 542                 /* Self-energy correction */
 543                 Vself_q[q] = ewc_q*one_4pi_eps*fr->q2sum[q]*M_1_SQRTPI;
 544                 if (EVDW_PME(fr->vdwtype))
 545                 {
 546                     Vself_lj[q] = -pow(ewc_lj, 6)*fr->c6sum[q];
 547                 }
 548             }
 549
 550             /* Apply surface dipole correction:
 551              * correction = dipole_coeff * (dipole)^2
 552              */
 553             if (dipole_coeff != 0)
 554             {
 555                 if (ewald_geometry == eewg3D)
 556                 {
 557                     Vdipole[q] = dipole_coeff*iprod(mutot[q], mutot[q]);
 558                 }
 559                 else if (ewald_geometry == eewg3DC)
 560                 {
 561                     Vdipole[q] = dipole_coeff*mutot[q][ZZ]*mutot[q][ZZ];
 562                 }
 563             }
 564         }
 565     }
 566     if (!bFreeEnergy)
 567     {
 568         *Vcorr_q = Vdipole[0] - Vself_q[0] - Vexcl_q;
 569         if (EVDW_PME(fr->vdwtype))
 570         {
 571             *Vcorr_lj = -Vself_lj[0] - Vexcl_lj;
 572         }
 573     }
 574     else
 575     {
 576         *Vcorr_q = L1_q*(Vdipole[0] - Vself_q[0])
 577             + lambda_q*(Vdipole[1] - Vself_q[1])
 578             - Vexcl_q;
 579         *dvdlambda_q += Vdipole[1] - Vself_q[1]
 580             - (Vdipole[0] - Vself_q[0]) - dvdl_excl_q;
 581         if (EVDW_PME(fr->vdwtype))
 582         {
 583             *Vcorr_lj      = -(L1_lj*Vself_lj[0] + lambda_lj*Vself_lj[1]) - Vexcl_lj;
 584             *dvdlambda_lj += -Vself_lj[1] + Vself_lj[0] - dvdl_excl_lj;
 585         }
 586     }
 587
 588     if (debug)
 589     {
 590         fprintf(debug, "Long Range corrections for Ewald interactions:\n");
 591         fprintf(debug, "start=%d,natoms=%d\n", start, end-start);
 592         fprintf(debug, "q2sum = %g, Vself_q=%g c6sum = %g, Vself_lj=%g\n",
 593                 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]);
 594         fprintf(debug, "Electrostatic Long Range correction: Vexcl=%g\n", Vexcl_q);
 595         fprintf(debug, "Lennard-Jones Long Range correction: Vexcl=%g\n", Vexcl_lj);
 596         if (MASTER(cr) && thread == 0)
 597         {
 598             if (epsilon_surface > 0 || ewald_geometry == eewg3DC)
 599             {
 600                 fprintf(debug, "Total dipole correction: Vdipole=%g\n",
 601                         L1_q*Vdipole[0]+lambda_q*Vdipole[1]);
 602             }
 603         }
 604     }
 605 }
 606
 607 real ewald_charge_correction(t_commrec *cr, t_forcerec *fr, real lambda,
 608                              matrix box,
 609                              real *dvdlambda, tensor vir)
 610
 611 {
 612     real vol, fac, qs2A, qs2B, vc, enercorr;
 613     int  d;
 614
 615     if (MASTER(cr))
 616     {
 617         /* Apply charge correction */
 618         vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 619
 620         fac = M_PI*ONE_4PI_EPS0/(fr->epsilon_r*2.0*vol*vol*sqr(fr->ewaldcoeff_q));
 621
 622         qs2A = fr->qsum[0]*fr->qsum[0];
 623         qs2B = fr->qsum[1]*fr->qsum[1];
 624
 625         vc = (qs2A*(1 - lambda) + qs2B*lambda)*fac;
 626
 627         enercorr = -vol*vc;
 628
 629         *dvdlambda += -vol*(qs2B - qs2A)*fac;
 630
 631         for (d = 0; d < DIM; d++)
 632         {
 633             vir[d][d] += vc;
 634         }
 635
 636         if (debug)
 637         {
 638             fprintf(debug, "Total charge correction: Vcharge=%g\n", enercorr);
 639         }
 640     }
 641     else
 642     {
 643         enercorr = 0;
 644     }
 645
 646     return enercorr;
 647 }