src/nonbonded/MAW.cpp

/*
 * Copyright (c) 2005 The University of Notre Dame. All Rights Reserved.
 *
 * The University of Notre Dame grants you ("Licensee") a
 * non-exclusive, royalty free, license to use, modify and
 * redistribute this software in source and binary code form, provided
 * that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the
 *    distribution.
 *
 * This software is provided "AS IS," without a warranty of any
 * kind. All express or implied conditions, representations and
 * warranties, including any implied warranty of merchantability,
 * fitness for a particular purpose or non-infringement, are hereby
 * excluded.  The University of Notre Dame and its licensors shall not
 * be liable for any damages suffered by licensee as a result of
 * using, modifying or distributing the software or its
 * derivatives. In no event will the University of Notre Dame or its
 * licensors be liable for any lost revenue, profit or data, or for
 * direct, indirect, special, consequential, incidental or punitive
 * damages, however caused and regardless of the theory of liability,
 * arising out of the use of or inability to use software, even if the
 * University of Notre Dame has been advised of the possibility of
 * such damages.
 *
 * SUPPORT OPEN SCIENCE!  If you use OpenMD or its source code in your
 * research, please cite the appropriate papers when you publish your
 * work.  Good starting points are:
 *                                                                      
 * [1]  Meineke, et al., J. Comp. Chem. 26, 252-271 (2005).             
 * [2]  Fennell & Gezelter, J. Chem. Phys. 124, 234104 (2006).          
 * [3]  Sun, Lin & Gezelter, J. Chem. Phys. 128, 234107 (2008).          
 * [4]  Kuang & Gezelter,  J. Chem. Phys. 133, 164101 (2010).
 * [5]  Vardeman, Stocker & Gezelter, J. Chem. Theory Comput. 7, 834 (2011).
 */

#include <stdio.h>
#include <string.h>
#include <cmath>

#include "nonbonded/MAW.hpp"
#include "utils/simError.h"

using namespace std;

namespace OpenMD {

  MAW::MAW() : name_("MAW"), initialized_(false), forceField_(NULL) {}
  
  void MAW::initialize() {    

    MAWtypes.clear();
    MAWtids.clear();
    MixingMap.clear();
    MAWtids.resize( forceField_->getNAtomType(), -1);

    ForceField::NonBondedInteractionTypeContainer* nbiTypes = forceField_->getNonBondedInteractionTypes();
    ForceField::NonBondedInteractionTypeContainer::MapTypeIterator j;
    ForceField::NonBondedInteractionTypeContainer::KeyType keys;
    NonBondedInteractionType* nbt;
    int mtid1, mtid2;

    for (nbt = nbiTypes->beginType(j); nbt != NULL; 
         nbt = nbiTypes->nextType(j)) {
      
      if (nbt->isMAW()) {
        keys = nbiTypes->getKeys(j);
        AtomType* at1 = forceField_->getAtomType(keys[0]);
        AtomType* at2 = forceField_->getAtomType(keys[1]);

        int atid1 = at1->getIdent();
        if (MAWtids[atid1] == -1) {
          mtid1 = MAWtypes.size();
          MAWtypes.insert(atid1);
          MAWtids[atid1] = mtid1;         
        }
        int atid2 = at2->getIdent();
        if (MAWtids[atid2] == -1) {
          mtid2 = MAWtypes.size();
          MAWtypes.insert(atid2);
          MAWtids[atid2] = mtid2;
        }

        MAWInteractionType* mit = dynamic_cast<MAWInteractionType*>(nbt);

        if (mit == NULL) {
          sprintf( painCave.errMsg,
                   "MAW::initialize could not convert NonBondedInteractionType\n"
                   "\tto MAWInteractionType for %s - %s interaction.\n", 
                   at1->getName().c_str(),
                   at2->getName().c_str());
          painCave.severity = OPENMD_ERROR;
          painCave.isFatal = 1;
          simError();          
        }
        
        RealType De = mit->getD();
        RealType beta = mit->getBeta();
        RealType Re = mit->getR();
        RealType ca1 = mit->getCA1();
        RealType cb1 = mit->getCB1();
        
        addExplicitInteraction(at1, at2, 
                               De, beta, Re, ca1, cb1);
      }
    }  
    initialized_ = true;
  }
  
  void MAW::addExplicitInteraction(AtomType* atype1, AtomType* atype2, 
                                   RealType De, RealType beta, RealType Re, 
                                   RealType ca1, RealType cb1) {

    MAWInteractionData mixer;
    mixer.De = De;
    mixer.beta = beta;
    mixer.Re = Re;
    mixer.ca1 = ca1;
    mixer.cb1 = cb1;
    mixer.j_is_Metal = atype2->isMetal();

    int mtid1 = MAWtids[atype1->getIdent()];
    int mtid2 = MAWtids[atype2->getIdent()];
    int nM = MAWtypes.size();

    MixingMap.resize(nM);
    MixingMap[mtid1].resize(nM);
    
    MixingMap[mtid1][mtid2] = mixer;
    if (mtid2 != mtid1) {
      MixingMap[mtid2].resize(nM);
      mixer.j_is_Metal = atype1->isMetal();
      MixingMap[mtid2][mtid1] = mixer;
    }    
  }
  
  void MAW::calcForce(InteractionData &idat) {

    if (!initialized_) initialize();

    MAWInteractionData &mixer = MixingMap[MAWtids[idat.atid1]][MAWtids[idat.atid2]];

    RealType myPot = 0.0;
    RealType myPotC = 0.0;
    RealType myDeriv = 0.0;
    RealType myDerivC = 0.0;
    
    RealType D_e = mixer.De;
    RealType R_e = mixer.Re;
    RealType beta = mixer.beta;
    RealType ca1 = mixer.ca1;
    RealType cb1 = mixer.cb1;   
    
    Vector3d r;
    RotMat3x3d Atrans;
    if (mixer.j_is_Metal) {
      // rotate the inter-particle separation into the two different 
      // body-fixed coordinate systems:
      r = *(idat.A1) * *(idat.d);
      Atrans = idat.A1->transpose();
    } else {
      // negative sign because this is the vector from j to i:       
      r = -*(idat.A2) * *(idat.d);
      Atrans = idat.A2->transpose();
    }
    
    // V(r) = D_e exp(-a(r-re)(exp(-a(r-re))-2)
    
    RealType expt     = -beta*( *(idat.rij) - R_e);
    RealType expfnc   = exp(expt);
    RealType expfnc2  = expfnc*expfnc;
    
    RealType exptC = 0.0;
    RealType expfncC = 0.0;
    RealType expfnc2C = 0.0;
    
    myPot  = D_e * (expfnc2  - 2.0 * expfnc);
    myDeriv   = 2.0 * D_e * beta * (expfnc - expfnc2);
    
    if (idat.shiftedPot || idat.shiftedForce) {
      exptC     = -beta*( *(idat.rcut)  - R_e);
      expfncC   = exp(exptC);
      expfnc2C  = expfncC*expfncC;
    }
    
    if (idat.shiftedPot) {
      myPotC = D_e * (expfnc2C - 2.0 * expfncC);
      myDerivC = 0.0;
    } else if (idat.shiftedForce) {
      myPotC = D_e * (expfnc2C - 2.0 * expfncC);
      myDerivC  = 2.0 * D_e * beta * (expfncC - expfnc2C);
      myPotC += myDerivC * ( *(idat.rij)  -  *(idat.rcut) );
    } else {
      myPotC = 0.0;
      myDerivC = 0.0;
    }
    
    RealType x = r.x();
    RealType y = r.y();
    RealType z = r.z();
    RealType x2 = x * x;
    RealType z2 = z * z;
    
    RealType r3 = *(idat.r2) *  *(idat.rij) ;
    RealType r4 = *(idat.r2) *  *(idat.r2);
    
    // angular modulation of morse part of potential to approximate 
    // the squares of the two HOMO lone pair orbitals in water:
    //********************** old form*************************
    // s = 1 / (4 pi)
    // ta1 = (s - pz)^2 = (1 - sqrt(3)*cos(theta))^2 / (4 pi)
    // b1 = px^2 = 3 * (sin(theta)*cos(phi))^2 / (4 pi)   
    //********************** old form*************************
    // we'll leave out the 4 pi for now
    
    // new functional form just using the p orbitals.
    // Vmorse(r)*[a*p_x + b p_z + (1-a-b)]
    // which is 
    // Vmorse(r)*[a sin^2(theta) cos^2(phi) + b cos(theta) + (1-a-b)]
    // Vmorse(r)*[a*x2/r2 + b*z/r + (1-a-b)]
    
    RealType Vmorse = (myPot - myPotC);
    RealType Vang = ca1 * x2 / *(idat.r2) + 
      cb1 * z /  *(idat.rij)  + (0.8 - ca1 / 3.0);
    
    RealType pot_temp = *(idat.vdwMult) * Vmorse * Vang;
    *(idat.vpair) += pot_temp;
    (*(idat.pot))[VANDERWAALS_FAMILY] += *(idat.sw) * pot_temp;
    
    Vector3d dVmorsedr = (myDeriv - myDerivC) * Vector3d(x, y, z) /  *(idat.rij) ;
    
    Vector3d dVangdr = Vector3d(-2.0 * ca1 * x2 * x / r4 + 2.0 * ca1 * x / *(idat.r2) - cb1 * x * z / r3,
                                -2.0 * ca1 * x2 * y / r4 - cb1 * z * y / r3,
                                -2.0 * ca1 * x2 * z / r4 + cb1 /  *(idat.rij)           - cb1 * z2  / r3);
    
    // chain rule to put these back on x, y, z
    
    Vector3d dvdr = Vang * dVmorsedr + Vmorse * dVangdr;
    
    // Torques for Vang using method of Price:
    // S. L. Price, A. J. Stone, and M. Alderton, Mol. Phys. 52, 987 (1984).
    
    Vector3d dVangdu = Vector3d(cb1 * y /  *(idat.rij) ,
                                2.0 * ca1 * x * z / *(idat.r2) - cb1 * x /  *(idat.rij),
                                -2.0 * ca1 * y * x / *(idat.r2));
    
    // do the torques first since they are easy:
    // remember that these are still in the body fixed axes    
    
    Vector3d trq = *(idat.vdwMult) * Vmorse * dVangdu * *(idat.sw);
    
    // go back to lab frame using transpose of rotation matrix:
    
    if (mixer.j_is_Metal) {
      *(idat.t1) += Atrans * trq;
    } else {
      *(idat.t2) += Atrans * trq;
    }
    
    // Now, on to the forces (still in body frame of water)
    
    Vector3d ftmp = *(idat.vdwMult) * *(idat.sw) * dvdr;
    
    // rotate the terms back into the lab frame:
    Vector3d flab;
    if (mixer.j_is_Metal) {
      flab = Atrans * ftmp;
    } else {
      flab = - Atrans * ftmp;
    }
    
    *(idat.f1) += flab;
    
    return;    
  }

  RealType MAW::getSuggestedCutoffRadius(pair<AtomType*, AtomType*> atypes) {
    if (!initialized_) initialize();   
    int atid1 = atypes.first->getIdent();
    int atid2 = atypes.second->getIdent();
    int mtid1 = MAWtids[atid1];
    int mtid2 = MAWtids[atid2];
    
    if ( mtid1 == -1 || mtid2 == -1) return 0.0;
    else {      
      MAWInteractionData mixer = MixingMap[mtid1][mtid2];
      RealType R_e = mixer.Re;
      RealType beta = mixer.beta;
      // This value of the r corresponds to an energy about 1.48% of 
      // the energy at the bottom of the Morse well.  For comparison, the
      // Lennard-Jones function is about 1.63% of it's minimum value at
      // a distance of 2.5 sigma.
      return (4.9 + beta * R_e) / beta;
    }
  }
}

Revision:	1930
Committed:	Mon Aug 19 13:51:04 2013 UTC (11 years, 11 months ago) by gezelter
File size:	10475 byte(s)
Log Message:	region fixes, performance boosts
#	User	Rev	Content
1	gezelter	1532	/*
2			* Copyright (c) 2005 The University of Notre Dame. All Rights Reserved.
3			*
4			* The University of Notre Dame grants you ("Licensee") a
5			* non-exclusive, royalty free, license to use, modify and
6			* redistribute this software in source and binary code form, provided
7			* that the following conditions are met:
8			*
9			* 1. Redistributions of source code must retain the above copyright
10			* notice, this list of conditions and the following disclaimer.
11			*
12			* 2. Redistributions in binary form must reproduce the above copyright
13			* notice, this list of conditions and the following disclaimer in the
14			* documentation and/or other materials provided with the
15			* distribution.
16			*
17			* This software is provided "AS IS," without a warranty of any
18			* kind. All express or implied conditions, representations and
19			* warranties, including any implied warranty of merchantability,
20			* fitness for a particular purpose or non-infringement, are hereby
21			* excluded. The University of Notre Dame and its licensors shall not
22			* be liable for any damages suffered by licensee as a result of
23			* using, modifying or distributing the software or its
24			* derivatives. In no event will the University of Notre Dame or its
25			* licensors be liable for any lost revenue, profit or data, or for
26			* direct, indirect, special, consequential, incidental or punitive
27			* damages, however caused and regardless of the theory of liability,
28			* arising out of the use of or inability to use software, even if the
29			* University of Notre Dame has been advised of the possibility of
30			* such damages.
31			*
32			* SUPPORT OPEN SCIENCE! If you use OpenMD or its source code in your
33			* research, please cite the appropriate papers when you publish your
34			* work. Good starting points are:
35			*
36			* [1] Meineke, et al., J. Comp. Chem. 26, 252-271 (2005).
37			* [2] Fennell & Gezelter, J. Chem. Phys. 124, 234104 (2006).
38	gezelter	1879	* [3] Sun, Lin & Gezelter, J. Chem. Phys. 128, 234107 (2008).
39	gezelter	1665	* [4] Kuang & Gezelter, J. Chem. Phys. 133, 164101 (2010).
40			* [5] Vardeman, Stocker & Gezelter, J. Chem. Theory Comput. 7, 834 (2011).
41	gezelter	1532	*/
42
43			#include <stdio.h>
44			#include <string.h>
45			#include <cmath>
46
47			#include "nonbonded/MAW.hpp"
48			#include "utils/simError.h"
49
50			using namespace std;
51
52			namespace OpenMD {
53
54	gezelter	1583	MAW::MAW() : name_("MAW"), initialized_(false), forceField_(NULL) {}
55	gezelter	1532
56			void MAW::initialize() {
57
58	gezelter	1895	MAWtypes.clear();
59			MAWtids.clear();
60			MixingMap.clear();
61			MAWtids.resize( forceField_->getNAtomType(), -1);
62
63	gezelter	1532	ForceField::NonBondedInteractionTypeContainer* nbiTypes = forceField_->getNonBondedInteractionTypes();
64			ForceField::NonBondedInteractionTypeContainer::MapTypeIterator j;
65	gezelter	1895	ForceField::NonBondedInteractionTypeContainer::KeyType keys;
66	gezelter	1532	NonBondedInteractionType* nbt;
67	gezelter	1895	int mtid1, mtid2;
68	gezelter	1532
69			for (nbt = nbiTypes->beginType(j); nbt != NULL;
70			nbt = nbiTypes->nextType(j)) {
71
72			if (nbt->isMAW()) {
73	gezelter	1664	keys = nbiTypes->getKeys(j);
74			AtomType* at1 = forceField_->getAtomType(keys[0]);
75			AtomType* at2 = forceField_->getAtomType(keys[1]);
76
77	gezelter	1895	int atid1 = at1->getIdent();
78			if (MAWtids[atid1] == -1) {
79			mtid1 = MAWtypes.size();
80			MAWtypes.insert(atid1);
81			MAWtids[atid1] = mtid1;
82			}
83			int atid2 = at2->getIdent();
84			if (MAWtids[atid2] == -1) {
85			mtid2 = MAWtypes.size();
86			MAWtypes.insert(atid2);
87			MAWtids[atid2] = mtid2;
88			}
89
90	gezelter	1664	MAWInteractionType* mit = dynamic_cast<MAWInteractionType*>(nbt);
91
92			if (mit == NULL) {
93	gezelter	1532	sprintf( painCave.errMsg,
94	gezelter	1664	"MAW::initialize could not convert NonBondedInteractionType\n"
95			"\tto MAWInteractionType for %s - %s interaction.\n",
96			at1->getName().c_str(),
97			at2->getName().c_str());
98	gezelter	1532	painCave.severity = OPENMD_ERROR;
99			painCave.isFatal = 1;
100			simError();
101			}
102
103	gezelter	1664	RealType De = mit->getD();
104			RealType beta = mit->getBeta();
105			RealType Re = mit->getR();
106			RealType ca1 = mit->getCA1();
107			RealType cb1 = mit->getCB1();
108	gezelter	1532
109	gezelter	1664	addExplicitInteraction(at1, at2,
110	gezelter	1532	De, beta, Re, ca1, cb1);
111			}
112			}
113			initialized_ = true;
114			}
115
116			void MAW::addExplicitInteraction(AtomType* atype1, AtomType* atype2,
117			RealType De, RealType beta, RealType Re,
118			RealType ca1, RealType cb1) {
119
120			MAWInteractionData mixer;
121			mixer.De = De;
122			mixer.beta = beta;
123			mixer.Re = Re;
124			mixer.ca1 = ca1;
125			mixer.cb1 = cb1;
126	gezelter	1930	mixer.j_is_Metal = atype2->isMetal();
127	gezelter	1532
128	gezelter	1895	int mtid1 = MAWtids[atype1->getIdent()];
129			int mtid2 = MAWtids[atype2->getIdent()];
130			int nM = MAWtypes.size();
131
132			MixingMap.resize(nM);
133			MixingMap[mtid1].resize(nM);
134	gezelter	1532
135	gezelter	1895	MixingMap[mtid1][mtid2] = mixer;
136			if (mtid2 != mtid1) {
137			MixingMap[mtid2].resize(nM);
138	gezelter	1930	mixer.j_is_Metal = atype1->isMetal();
139	gezelter	1895	MixingMap[mtid2][mtid1] = mixer;
140	gezelter	1532	}
141			}
142
143	gezelter	1536	void MAW::calcForce(InteractionData &idat) {
144	gezelter	1532
145			if (!initialized_) initialize();
146
147	gezelter	1895	MAWInteractionData &mixer = MixingMap[MAWtids[idat.atid1]][MAWtids[idat.atid2]];
148	gezelter	1532
149	gezelter	1895	RealType myPot = 0.0;
150			RealType myPotC = 0.0;
151			RealType myDeriv = 0.0;
152			RealType myDerivC = 0.0;
153
154			RealType D_e = mixer.De;
155			RealType R_e = mixer.Re;
156			RealType beta = mixer.beta;
157			RealType ca1 = mixer.ca1;
158	gezelter	1930	RealType cb1 = mixer.cb1;
159	gezelter	1895
160			Vector3d r;
161			RotMat3x3d Atrans;
162	gezelter	1930	if (mixer.j_is_Metal) {
163	gezelter	1895	// rotate the inter-particle separation into the two different
164			// body-fixed coordinate systems:
165			r = (idat.A1) *(idat.d);
166			Atrans = idat.A1->transpose();
167			} else {
168			// negative sign because this is the vector from j to i:
169			r = -(idat.A2) *(idat.d);
170			Atrans = idat.A2->transpose();
171	gezelter	1532	}
172
173	gezelter	1895	// V(r) = D_e exp(-a(r-re)(exp(-a(r-re))-2)
174
175			RealType expt = -beta( (idat.rij) - R_e);
176			RealType expfnc = exp(expt);
177			RealType expfnc2 = expfnc*expfnc;
178
179			RealType exptC = 0.0;
180			RealType expfncC = 0.0;
181			RealType expfnc2C = 0.0;
182
183			myPot = D_e * (expfnc2 - 2.0 * expfnc);
184			myDeriv = 2.0 * D_e * beta * (expfnc - expfnc2);
185
186			if (idat.shiftedPot \|\| idat.shiftedForce) {
187			exptC = -beta( (idat.rcut) - R_e);
188			expfncC = exp(exptC);
189			expfnc2C = expfncC*expfncC;
190			}
191
192			if (idat.shiftedPot) {
193			myPotC = D_e * (expfnc2C - 2.0 * expfncC);
194			myDerivC = 0.0;
195			} else if (idat.shiftedForce) {
196			myPotC = D_e * (expfnc2C - 2.0 * expfncC);
197			myDerivC = 2.0 * D_e * beta * (expfncC - expfnc2C);
198			myPotC += myDerivC * ( (idat.rij) - (idat.rcut) );
199			} else {
200			myPotC = 0.0;
201			myDerivC = 0.0;
202			}
203
204			RealType x = r.x();
205			RealType y = r.y();
206			RealType z = r.z();
207			RealType x2 = x * x;
208			RealType z2 = z * z;
209
210			RealType r3 = (idat.r2) *(idat.rij) ;
211			RealType r4 = (idat.r2) *(idat.r2);
212
213			// angular modulation of morse part of potential to approximate
214			// the squares of the two HOMO lone pair orbitals in water:
215			//******************** old form***********************
216			// s = 1 / (4 pi)
217			// ta1 = (s - pz)^2 = (1 - sqrt(3)*cos(theta))^2 / (4 pi)
218			// b1 = px^2 = 3 * (sin(theta)*cos(phi))^2 / (4 pi)
219			//******************** old form***********************
220			// we'll leave out the 4 pi for now
221
222			// new functional form just using the p orbitals.
223			// Vmorse(r)[ap_x + b p_z + (1-a-b)]
224			// which is
225			// Vmorse(r)*[a sin^2(theta) cos^2(phi) + b cos(theta) + (1-a-b)]
226			// Vmorse(r)[ax2/r2 + b*z/r + (1-a-b)]
227
228			RealType Vmorse = (myPot - myPotC);
229			RealType Vang = ca1 * x2 / *(idat.r2) +
230			cb1 * z / *(idat.rij) + (0.8 - ca1 / 3.0);
231
232			RealType pot_temp = (idat.vdwMult) Vmorse * Vang;
233			*(idat.vpair) += pot_temp;
234			((idat.pot))[VANDERWAALS_FAMILY] += (idat.sw) * pot_temp;
235
236			Vector3d dVmorsedr = (myDeriv - myDerivC) * Vector3d(x, y, z) / *(idat.rij) ;
237
238			Vector3d dVangdr = Vector3d(-2.0 * ca1 * x2 * x / r4 + 2.0 * ca1 * x / (idat.r2) - cb1 x * z / r3,
239			-2.0 * ca1 * x2 * y / r4 - cb1 * z * y / r3,
240			-2.0 * ca1 * x2 * z / r4 + cb1 / (idat.rij) - cb1 z2 / r3);
241
242			// chain rule to put these back on x, y, z
243
244			Vector3d dvdr = Vang * dVmorsedr + Vmorse * dVangdr;
245
246			// Torques for Vang using method of Price:
247			// S. L. Price, A. J. Stone, and M. Alderton, Mol. Phys. 52, 987 (1984).
248
249			Vector3d dVangdu = Vector3d(cb1 * y / *(idat.rij) ,
250			2.0 * ca1 * x * z / (idat.r2) - cb1 x / *(idat.rij),
251			-2.0 * ca1 * y * x / *(idat.r2));
252
253			// do the torques first since they are easy:
254			// remember that these are still in the body fixed axes
255
256			Vector3d trq = (idat.vdwMult) Vmorse * dVangdu * *(idat.sw);
257
258			// go back to lab frame using transpose of rotation matrix:
259
260	gezelter	1930	if (mixer.j_is_Metal) {
261	gezelter	1895	(idat.t1) += Atrans trq;
262			} else {
263			(idat.t2) += Atrans trq;
264			}
265
266			// Now, on to the forces (still in body frame of water)
267
268			Vector3d ftmp = (idat.vdwMult) (idat.sw) dvdr;
269
270			// rotate the terms back into the lab frame:
271			Vector3d flab;
272	gezelter	1930	if (mixer.j_is_Metal) {
273	gezelter	1895	flab = Atrans * ftmp;
274			} else {
275			flab = - Atrans * ftmp;
276			}
277
278			*(idat.f1) += flab;
279
280			return;
281	gezelter	1532	}
282	gezelter	1895
283	gezelter	1545	RealType MAW::getSuggestedCutoffRadius(pair<AtomType, AtomType> atypes) {
284	gezelter	1532	if (!initialized_) initialize();
285	gezelter	1895	int atid1 = atypes.first->getIdent();
286			int atid2 = atypes.second->getIdent();
287			int mtid1 = MAWtids[atid1];
288			int mtid2 = MAWtids[atid2];
289
290			if ( mtid1 == -1 \|\| mtid2 == -1) return 0.0;
291			else {
292			MAWInteractionData mixer = MixingMap[mtid1][mtid2];
293	gezelter	1532	RealType R_e = mixer.Re;
294			RealType beta = mixer.beta;
295			// This value of the r corresponds to an energy about 1.48% of
296			// the energy at the bottom of the Morse well. For comparison, the
297			// Lennard-Jones function is about 1.63% of it's minimum value at
298			// a distance of 2.5 sigma.
299			return (4.9 + beta * R_e) / beta;
300			}
301			}
302			}
303