X-Git-Url: https://www.hackdaworld.org/gitweb/?a=blobdiff_plain;f=moldyn.c;h=716a087fa90f1b89b073411af22e2d55995aba8a;hb=15dd7ee084a5db24a3e1a15428cd47e8d61fda5b;hp=d52fcd3868c2481e70cbc804b5a9cdf92542708d;hpb=180ff86ae35ca0ea6ee857e92e56120e65315d17;p=physik%2Fposic.git

diff --git a/moldyn.c b/moldyn.c
index d52fcd3..716a087 100644
--- a/moldyn.c
+++ b/moldyn.c
@@ -71,9 +71,18 @@ int set_cutoff(t_moldyn *moldyn,double cutoff) {
 	return 0;
 }
 
-int set_temperature(t_moldyn *moldyn,double t) {
-	
-	moldyn->t=t;
+int set_temperature(t_moldyn *moldyn,double t_ref) {
+
+	moldyn->t_ref=t_ref;
+
+	return 0;
+}
+
+int set_pt_scale(t_moldyn *moldyn,u8 ptype,double ptc,u8 ttype,double ttc) {
+
+	moldyn->pt_scale=(ptype|ttype);
+	moldyn->t_tc=ttc;
+	moldyn->p_tc=ptc;
 
 	return 0;
 }
@@ -130,6 +139,14 @@ int set_potential2b(t_moldyn *moldyn,pf_func2b func,void *params) {
 	return 0;
 }
 
+int set_potential2b_post(t_moldyn *moldyn,pf_func2b_post func,void *params) {
+
+	moldyn->func2b=func;
+	moldyn->pot2b_params=params;
+
+	return 0;
+}
+
 int set_potential3b(t_moldyn *moldyn,pf_func3b func,void *params) {
 
 	moldyn->func3b=func;
@@ -231,13 +248,15 @@ int create_lattice(t_moldyn *moldyn,u8 type,double lc,int element,double mass,
 	printf("[moldyn] created lattice with %d atoms\n",count);
 
 	while(count) {
-		moldyn->atom[count-1].element=element;
-		moldyn->atom[count-1].mass=mass;
-		moldyn->atom[count-1].attr=attr;
-		moldyn->atom[count-1].bnum=bnum;
 		count-=1;
+		moldyn->atom[count].element=element;
+		moldyn->atom[count].mass=mass;
+		moldyn->atom[count].attr=attr;
+		moldyn->atom[count].bnum=bnum;
+		check_per_bound(moldyn,&(moldyn->atom[count].r));
 	}
 
+
 	return ret;
 }
 
@@ -276,7 +295,7 @@ int destroy_atoms(t_moldyn *moldyn) {
 	return 0;
 }
 
-int thermal_init(t_moldyn *moldyn) {
+int thermal_init(t_moldyn *moldyn,u8 equi_init) {
 
 	/*
 	 * - gaussian distribution of velocities
@@ -296,7 +315,7 @@ int thermal_init(t_moldyn *moldyn) {
 	/* gaussian distribution of velocities */
 	v3_zero(&p_total);
 	for(i=0;i<moldyn->count;i++) {
-		sigma=sqrt(2.0*K_BOLTZMANN*moldyn->t/atom[i].mass);
+		sigma=sqrt(2.0*K_BOLTZMANN*moldyn->t_ref/atom[i].mass);
 		/* x direction */
 		v=sigma*rand_get_gauss(random);
 		atom[i].v.x=v;
@@ -319,16 +338,17 @@ int thermal_init(t_moldyn *moldyn) {
 	}
 
 	/* velocity scaling */
-	scale_velocity(moldyn,VSCALE_INIT_EQUI);
+	scale_velocity(moldyn,equi_init);
 
 	return 0;
 }
 
-int scale_velocity(t_moldyn *moldyn,u8 type) {
+int scale_velocity(t_moldyn *moldyn,u8 equi_init) {
 
 	int i;
 	double e,scale;
 	t_atom *atom;
+	int count;
 
 	atom=moldyn->atom;
 
@@ -336,14 +356,41 @@ int scale_velocity(t_moldyn *moldyn,u8 type) {
 	 * - velocity scaling (E = 3/2 N k T), E: kinetic energy
 	 */
 
+	/* get kinetic energy / temperature & count involved atoms */
 	e=0.0;
-	for(i=0;i<moldyn->count;i++)
-		e+=0.5*atom[i].mass*v3_absolute_square(&(atom[i].v));
-	scale=(1.5*moldyn->count*K_BOLTZMANN*moldyn->t)/e;
-	if(type&VSCALE_INIT_EQUI) scale*=2.0; /* equipartition theorem */
+	count=0;
+	for(i=0;i<moldyn->count;i++) {
+		if((equi_init&TRUE)||(atom[i].attr&ATOM_ATTR_HB)) {
+			e+=0.5*atom[i].mass*v3_absolute_square(&(atom[i].v));
+			count+=1;
+		}
+	}
+	if(count!=0) moldyn->t=(2.0*e)/(3.0*count*K_BOLTZMANN);
+	else return 0;	/* no atoms involved in scaling! */
+	
+	/* (temporary) hack for e,t = 0 */
+	if(e==0.0) {
+	moldyn->t=0.0;
+		if(moldyn->t_ref!=0.0)
+			thermal_init(moldyn,equi_init);
+		else
+			return 0; /* no scaling needed */
+	}
+
+
+	/* get scaling factor */
+	scale=moldyn->t_ref/moldyn->t;
+	if(equi_init&TRUE)
+		scale*=2.0;
+	else
+		if(moldyn->pt_scale&T_SCALE_BERENDSEN)
+			scale=1.0+moldyn->tau*(scale-1.0)/moldyn->t_tc;
 	scale=sqrt(scale);
+
+	/* velocity scaling */
 	for(i=0;i<moldyn->count;i++)
-		v3_scale(&(atom[i].v),&(atom[i].v),scale);
+		if((equi_init&TRUE)||(atom[i].attr&ATOM_ATTR_HB))
+			v3_scale(&(atom[i].v),&(atom[i].v),scale);
 
 	return 0;
 }
@@ -495,7 +542,6 @@ int link_cell_neighbour_index(t_moldyn *moldyn,int i,int j,int k,t_list *cell) {
 	count2=27;
 	a=nx*ny;
 
-
 	cell[0]=lc->subcell[i+j*nx+k*a];
 	for(ci=-1;ci<=1;ci++) {
 		bx=0;
@@ -529,7 +575,7 @@ int link_cell_neighbour_index(t_moldyn *moldyn,int i,int j,int k,t_list *cell) {
 		}
 	}
 
-	lc->dnlc=count2;
+	lc->dnlc=count1;
 	lc->countn=27;
 
 	return count2;
@@ -618,7 +664,6 @@ int moldyn_integrate(t_moldyn *moldyn) {
 	/* sqaure of some variables */
 	moldyn->tau_square=moldyn->tau*moldyn->tau;
 	moldyn->cutoff_square=moldyn->cutoff*moldyn->cutoff;
-
 	/* calculate initial forces */
 	potential_force_calc(moldyn);
 
@@ -635,7 +680,6 @@ int moldyn_integrate(t_moldyn *moldyn) {
 
 	/* zero absolute time */
 	moldyn->time=0.0;
-
 	for(sched=0;sched<moldyn->schedule.content_count;sched++) {
 
 		/* setting amount of runs and finite time step size */
@@ -650,6 +694,10 @@ int moldyn_integrate(t_moldyn *moldyn) {
 		/* integration step */
 		moldyn->integrate(moldyn);
 
+		/* p/t scaling */
+		if(moldyn->pt_scale&(T_SCALE_BERENDSEN|T_SCALE_DIRECT))
+			scale_velocity(moldyn,FALSE);
+
 		/* increase absolute time */
 		moldyn->time+=moldyn->tau;
 
@@ -759,107 +807,164 @@ int velocity_verlet(t_moldyn *moldyn) {
 int potential_force_calc(t_moldyn *moldyn) {
 
 	int i,j,k,count;
-	t_atom *atom,*btom,*ktom;
+	t_atom *itom,*jtom,*ktom;
 	t_linkcell *lc;
-	t_list neighbour[27];
-	t_list *this,*thisk,*neighbourk;
-	u8 bc,bck;
+	t_list neighbour_i[27];
+	t_list neighbour_i2[27];
+	//t_list neighbour_j[27];
+	t_list *this,*that;
+	u8 bc_ij,bc_ijk;
 	int countn,dnlc;
 
 	count=moldyn->count;
-	atom=moldyn->atom;
+	itom=moldyn->atom;
 	lc=&(moldyn->lc);
 
 	/* reset energy */
 	moldyn->energy=0.0;
 
 	for(i=0;i<count;i++) {
-	
+
 		/* reset force */
-		v3_zero(&(atom[i].f));
+		v3_zero(&(itom[i].f));
 
 		/* single particle potential/force */
-		if(atom[i].attr&ATOM_ATTR_1BP)
-			moldyn->func1b(moldyn,&(atom[i]));
+		if(itom[i].attr&ATOM_ATTR_1BP)
+			moldyn->func1b(moldyn,&(itom[i]));
 
 		/* 2 body pair potential/force */
-		if(atom[i].attr&(ATOM_ATTR_2BP|ATOM_ATTR_3BP)) {
+		if(itom[i].attr&(ATOM_ATTR_2BP|ATOM_ATTR_3BP)) {
 	
 			link_cell_neighbour_index(moldyn,
-				(atom[i].r.x+moldyn->dim.x/2)/lc->x,
-				(atom[i].r.y+moldyn->dim.y/2)/lc->y,
-				(atom[i].r.z+moldyn->dim.z/2)/lc->z,
-				neighbour);
+				(itom[i].r.x+moldyn->dim.x/2)/lc->x,
+				(itom[i].r.y+moldyn->dim.y/2)/lc->y,
+				(itom[i].r.z+moldyn->dim.z/2)/lc->z,
+				neighbour_i);
 
 			countn=lc->countn;
 			dnlc=lc->dnlc;
 
 			for(j=0;j<countn;j++) {
 
-				this=&(neighbour[j]);
+				this=&(neighbour_i[j]);
 				list_reset(this);
 
 				if(this->start==NULL)
 					continue;
 
-				bc=(j<dnlc)?0:1;
+				bc_ij=(j<dnlc)?0:1;
 
 				do {
-					btom=this->current->data;
+					jtom=this->current->data;
 
-					if(btom==&(atom[i]))
+					if(jtom==&(itom[i]))
 						continue;
 
-					if((btom->attr&ATOM_ATTR_2BP)&
-					   (atom[i].attr&ATOM_ATTR_2BP))
+					if((jtom->attr&ATOM_ATTR_2BP)&
+					   (itom[i].attr&ATOM_ATTR_2BP))
 						moldyn->func2b(moldyn,
-						               &(atom[i]),
-						               btom,
-						               bc);
+						               &(itom[i]),
+						               jtom,
+						               bc_ij);
 
 					/* 3 body potential/force */
 
-					if(!(atom[i].attr&ATOM_ATTR_3BP)||
-					   !(btom->attr&ATOM_ATTR_3BP))
+					if(!(itom[i].attr&ATOM_ATTR_3BP)||
+					   !(jtom->attr&ATOM_ATTR_3BP))
 						continue;
 
-					link_cell_neighbour_index(moldyn,
-					   (btom->r.x+moldyn->dim.x/2)/lc->x,
-					   (btom->r.y+moldyn->dim.y/2)/lc->y,
-					   (btom->r.z+moldyn->dim.z/2)/lc->z,
-					   neighbourk);
-
-					for(k=0;k<lc->countn;k++) {
-
-						thisk=&(neighbourk[k]);
-						list_reset(thisk);
+			/*
+			 * according to mr. nordlund, we dont need to take the 
+			 * sum over all atoms now, as 'this is centered' around
+			 * atom i ...
+			 * i am not quite sure though! there is a not vanishing
+			 * part even if f_c_ik is zero ...
+			 * this analytical potentials suck!
+			 * switching from mc to md to dft soon!
+			 */
+
+			//		link_cell_neighbour_index(moldyn,
+			//		   (jtom->r.x+moldyn->dim.x/2)/lc->x,
+			//		   (jtom->r.y+moldyn->dim.y/2)/lc->y,
+			//		   (jtom->r.z+moldyn->dim.z/2)/lc->z,
+			//		   neighbour_j);
+
+//					/* neighbours of j */
+//					for(k=0;k<lc->countn;k++) {
+//
+//						that=&(neighbour_j[k]);
+//						list_reset(that);
+//					
+//						if(that->start==NULL)
+//							continue;
+//
+//						bc_ijk=(k<lc->dnlc)?0:1;
+//
+//						do {
+//
+//			ktom=that->current->data;
+//
+//			if(!(ktom->attr&ATOM_ATTR_3BP))
+//				continue;
+//
+//			if(ktom==jtom)
+//				continue;
+//
+//			if(ktom==&(itom[i]))
+//				continue;
+//
+//			moldyn->func3b(moldyn,&(itom[i]),jtom,ktom,bc_ijk);
+//
+/*						} while(list_next(that)!=\ */
+//						        L_NO_NEXT_ELEMENT);
+//
+//					}
+			
+					/* copy the neighbour lists */
+					memcpy(neighbour_i2,neighbour_i,
+					       27*sizeof(t_list));
+
+					/* get neighbours of i */
+					for(k=0;k<countn;k++) {
+
+						that=&(neighbour_i2[k]);
+						list_reset(that);
 					
-						if(thisk->start==NULL)
+						if(that->start==NULL)
 							continue;
 
-						bck=(k<lc->dnlc)?0:1;
+						bc_ijk=(k<dnlc)?0:1;
 
 						do {
 
-			ktom=thisk->current->data;
+			ktom=that->current->data;
 
 			if(!(ktom->attr&ATOM_ATTR_3BP))
 				continue;
 
-			if(ktom==btom)
+			if(ktom==jtom)
 				continue;
 
-			if(ktom==&(atom[i]))
+			if(ktom==&(itom[i]))
 				continue;
 
-			moldyn->func3b(moldyn,&(atom[i]),btom,ktom,bck);
+//printf("Debug: atom %d before 3bp: %08x %08x %08x | %.15f %.15f %.15f\n",i,&itom[i],jtom,ktom,itom[i].r.x,itom[i].f.x,itom[i].v.x);
+			moldyn->func3b(moldyn,&(itom[i]),jtom,ktom,bc_ijk);
+//printf("Debug: atom %d after 3bp: %08x %08x %08x | %.15f %.15f %.15f\n",i,&itom[i],jtom,ktom,itom[i].r.x,itom[i].f.x,itom[i].v.x);
 
-						} while(list_next(thisk)!=\
+						} while(list_next(that)!=\
 						        L_NO_NEXT_ELEMENT);
 
 					}
 					
 				} while(list_next(this)!=L_NO_NEXT_ELEMENT);
+		
+				/* 2bp post function */
+				if(moldyn->func2b_post)
+					moldyn->func2b_post(moldyn,
+					                    &(itom[i]),
+					                    jtom,bc_ij);
+
 			}
 		}
 	}
@@ -971,6 +1076,35 @@ int lennard_jones(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
  * tersoff potential & force for 2 sorts of atoms
  */
 
+/* create mixed terms from parameters and set them */
+int tersoff_mult_complete_params(t_tersoff_mult_params *p) {
+
+	printf("[moldyn] tersoff parameter completion\n");
+	p->Smixed=sqrt(p->S[0]*p->S[1]);
+	p->Rmixed=sqrt(p->R[0]*p->R[1]);
+	p->Amixed=sqrt(p->A[0]*p->A[1]);
+	p->Bmixed=sqrt(p->B[0]*p->B[1]);
+	p->lambda_m=0.5*(p->lambda[0]+p->lambda[1]);
+	p->mu_m=0.5*(p->mu[0]+p->mu[1]);
+
+	printf("[moldyn] tersoff mult parameter info:\n");
+	printf("  S (m)  | %.12f | %.12f | %.12f\n",p->S[0],p->S[1],p->Smixed);
+	printf("  R (m)  | %.12f | %.12f | %.12f\n",p->R[0],p->R[1],p->Rmixed);
+	printf("  A (eV) | %f | %f | %f\n",p->A[0]/EV,p->A[1]/EV,p->Amixed/EV);
+	printf("  B (eV) | %f | %f | %f\n",p->B[0]/EV,p->B[1]/EV,p->Bmixed/EV);
+	printf("  lambda | %f | %f | %f\n",p->lambda[0],p->lambda[1],
+	                                  p->lambda_m);
+	printf("  mu     | %f | %f | %f\n",p->mu[0],p->mu[1],p->mu_m);
+	printf("  beta   | %.10f | %.10f\n",p->beta[0],p->beta[1]);
+	printf("  n      | %f | %f\n",p->n[0],p->n[1]);
+	printf("  c      | %f | %f\n",p->c[0],p->c[1]);
+	printf("  d      | %f | %f\n",p->d[0],p->d[1]);
+	printf("  h      | %f | %f\n",p->h[0],p->h[1]);
+	printf("  chi    | %f \n",p->chi);
+
+	return 0;
+}
+
 /* tersoff 1 body part */
 int tersoff_mult_1bp(t_moldyn *moldyn,t_atom *ai) {
 
@@ -994,6 +1128,7 @@ int tersoff_mult_1bp(t_moldyn *moldyn,t_atom *ai) {
 	exchange->h=&(params->h[num]);
 
 	exchange->betan=pow(*(exchange->beta),*(exchange->n));
+	exchange->n_betan=*(exchange->n)*exchange->betan;
 	exchange->c2=params->c[num]*params->c[num];
 	exchange->d2=params->d[num]*params->d[num];
 	exchange->c2d2=exchange->c2/exchange->d2;
@@ -1032,18 +1167,20 @@ int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 
 	if(bc) check_per_bound(moldyn,&dist_ij);
 
-	/* save for use in 3bp */ /* REALLY ?!?!?! */
+	d_ij=v3_norm(&dist_ij);
+
+	/* save for use in 3bp */
+	exchange->d_ij=d_ij;
 	exchange->dist_ij=dist_ij;
+	exchange->d_ij2=d_ij*d_ij;
 
 	/* constants */
 	if(num==aj->bnum) {
 		S=params->S[num];
 		R=params->R[num];
 		A=params->A[num];
+		B=params->B[num];
 		lambda=params->lambda[num];
-		/* more constants depending of atoms i and j, needed in 3bp */
-		params->exchange.B=&(params->B[num]);
-		params->exchange.mu=&(params->mu[num]);
 		mu=params->mu[num];
 		params->exchange.chi=1.0;
 	}
@@ -1051,19 +1188,12 @@ int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 		S=params->Smixed;
 		R=params->Rmixed;
 		A=params->Amixed;
+		B=params->Bmixed;
 		lambda=params->lambda_m;
-		/* more constants depending of atoms i and j, needed in 3bp */
-		params->exchange.B=&(params->Bmixed);
-		params->exchange.mu=&(params->mu_m);
 		mu=params->mu_m;
 		params->exchange.chi=params->chi;
 	}
 
-	d_ij=v3_norm(&dist_ij);
-
-	/* save for use in 3bp */
-	exchange->d_ij=d_ij;
-
 	if(d_ij>S)
 		return 0;
 
@@ -1091,8 +1221,8 @@ int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 
 	/* add forces */
 	v3_add(&(ai->f),&(ai->f),&force);
-	/* energy is 0.5 f_r f_c, but we will sum it up twice ... */
-	moldyn->energy+=(0.25*f_r*f_c);
+	/* energy is 0.5 f_r f_c ... */
+	moldyn->energy+=(0.5*f_r*f_c);
 
 	/* save for use in 3bp */
 	exchange->f_c=f_c;
@@ -1101,6 +1231,64 @@ int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 	/* enable the run of 3bp function */
 	exchange->run3bp=1;
 
+	/* reset 3bp sums */
+	exchange->sum1_3bp=0.0;
+	exchange->sum2_3bp=0.0;
+	v3_zero(&(exchange->db_ij));
+
+	return 0;
+}
+
+/* tersoff 2 body post part */
+
+int tersoff_mult_post_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
+
+	/* here we have to allow for the 3bp sums */
+
+	t_tersoff_mult_params *params;
+	t_tersoff_exchange *exchange;
+
+	t_3dvec force,temp,*db_ij,*dist_ij;
+	double db_ij_scale1,db_ij_scale2;
+	double b_ij;
+	double f_c,df_c,f_a,df_a;
+	double chi,betan;
+	double help;
+	double n;
+
+	params=moldyn->pot2b_params;
+	exchange=&(params->exchange);
+
+	db_ij=&(exchange->db_ij);
+	f_c=exchange->f_c;
+	df_c=exchange->df_c;
+	f_a=exchange->f_a;
+	df_a=exchange->df_a;
+	betan=exchange->betan;
+	n=*(exchange->n);
+	dist_ij=&(exchange->dist_ij);
+
+	db_ij_scale1=(1+betan*exchange->sum1_3bp);
+	db_ij_scale2=(exchange->n_betan*exchange->sum2_3bp);
+	help=pow(db_ij_scale1,-1.0/(2*n)-1);
+	b_ij=chi*db_ij_scale1*help;
+	db_ij_scale1=-chi/(2*n)*help;
+
+	v3_scale(db_ij,db_ij,(db_ij_scale1*db_ij_scale2));
+	v3_scale(db_ij,db_ij,f_a);
+
+	v3_scale(&temp,dist_ij,b_ij*df_a);
+
+	v3_add(&force,&temp,db_ij);
+	v3_scale(&force,&force,f_c);
+
+	v3_scale(&temp,dist_ij,f_a*b_ij*df_c);
+
+	/* add energy of 3bp sum */
+	moldyn->energy+=(0.5*f_c*b_ij*f_a);
+	/* add force of 3bp calculation */	
+	v3_add(&(ai->f),&temp,&force);
+
 	return 0;
 }
 
@@ -1114,18 +1302,16 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	t_3dvec temp,force;
 	double R,S,s_r;
 	double d_ij,d_ij2,d_ik,d_jk;
-	double f_c,df_c,b_ij,f_a,df_a;
+	double f_c,df_c,f_a,df_a;
 	double f_c_ik,df_c_ik,arg;
-	double scale;
-	double chi;
-	double n,c,d,h,beta,betan;
+	double n,c,d,h;
 	double c2,d2,c2d2;
 	double numer,denom;
 	double theta,cos_theta,sin_theta;
 	double d_theta,d_theta1,d_theta2;
-	double h_cos,h_cos2,d2_h_cos2;
-	double frac1,bracket1,bracket2,bracket2_n_1,bracket2_n;
-	double bracket3,bracket3_pow_1,bracket3_pow;
+	double h_cos,d2_h_cos2;
+	double frac,bracket,bracket_n_1,bracket_n;
+	double g;
 	int num;
 
 	params=moldyn->pot3b_params;
@@ -1143,9 +1329,13 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 
 	d_ij=exchange->d_ij;
 	d_ij2=exchange->d_ij2;
+	dist_ij=exchange->dist_ij;
 
 	f_a=params->exchange.f_a;
 	df_a=params->exchange.df_a;
+
+	f_c=exchange->f_c;
+	df_c=exchange->df_c;
 	
 	/* d_ij is <= S, as we didn't return so far! */
 
@@ -1159,7 +1349,6 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	 *
 	 */
 
-	
 	v3_sub(&dist_ik,&(ai->r),&(ak->r));
 	if(bc) check_per_bound(moldyn,&dist_ik);
 	d_ik=v3_norm(&dist_ik);
@@ -1175,11 +1364,11 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	}
 
 	/* calc of f_c_ik */
-	if(d_ik>S)
-		return 0;
-
-	if(d_ik<R) {
-		/* f_c_ik = 1, df_c_ik = 0 */
+	if(d_ik>S) {
+		f_c_ik=0.0;
+		df_c_ik=0.0;
+	}
+	else if(d_ik<R) {
 		f_c_ik=1.0;
 		df_c_ik=0.0;
 	}
@@ -1194,8 +1383,7 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	if(bc) check_per_bound(moldyn,&dist_jk);
 	d_jk=v3_norm(&dist_jk);
 
-	beta=*(exchange->beta);
-	betan=exchange->betan;
+	/* get exchange data */
 	n=*(exchange->n);
 	c=*(exchange->c);
 	d=*(exchange->d);
@@ -1207,6 +1395,8 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	numer=d_ij2+d_ik*d_ik-d_jk*d_jk;
 	denom=2*d_ij*d_ik;
 	cos_theta=numer/denom;
+	/* prefere law of cosines, dot product -> nan (often) */
+	//cos_theta=v3_scalar_product(&dist_ij,&dist_ik)/(d_ij*d_ik);
 	sin_theta=sqrt(1.0-(cos_theta*cos_theta));
 	theta=acos(cos_theta);
 	d_theta=(-1.0/sqrt(1.0-cos_theta*cos_theta))/(denom*denom);
@@ -1216,50 +1406,43 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	d_theta2*=d_theta;
 
 	h_cos=(h-cos_theta);
-	h_cos2=h_cos*h_cos;
-	d2_h_cos2=d2-h_cos2;
-
-	/* some usefull expressions */
-	frac1=c2/(d2-h_cos2);
-	bracket1=1+c2d2-frac1;
-	bracket2=f_c_ik*bracket1;
-	bracket2_n_1=pow(bracket2,n-1.0);
-	bracket2_n=bracket2_n_1*bracket2;
-	bracket3=1+betan*bracket2_n;
-	bracket3_pow_1=pow(bracket3,(-1.0/(2.0*n))-1.0);
-	bracket3_pow=bracket3_pow_1*bracket3;
-
-	/* now go on with calc of b_ij and derivation of b_ij */
-	b_ij=chi*bracket3_pow;
+	d2_h_cos2=d2+(h_cos*h_cos);
+
+	frac=c2/(d2_h_cos2);
+	g=1.0+c2d2-frac;
+
+	if(f_c_ik==0.0) {
+		bracket=0.0;
+		bracket_n_1=0.0;
+		bracket_n=0.0;
+		printf("Foo -> 0: ");
+	}
+	else {
+		bracket=f_c_ik*g;
+		bracket_n_1=pow(bracket,n-1.0);
+		bracket_n=bracket_n_1*bracket;
+		printf("Foo -> 1: ");
+	}
+//printf("%.15f %.15f %.15f\n",bracket_n_1,bracket_n,bracket);
+
+	/* calc of db_ij and the 2 sums */
+	exchange->sum1_3bp+=bracket_n;
+	exchange->sum2_3bp+=bracket_n_1;
 
 	/* derivation of theta */
 	v3_scale(&force,&dist_ij,d_theta1);
 	v3_scale(&temp,&dist_ik,d_theta2);
 	v3_add(&force,&force,&temp);
 
-	/* part 1 of derivation of b_ij */
-	v3_scale(&force,&force,sin_theta*2*h_cos*f_c_ik*frac1);
+	/* part 1 of db_ij */
+	v3_scale(&force,&force,sin_theta*2*h_cos*f_c_ik*frac/d2_h_cos2);
 
-	/* part 2 of derivation of b_ij */
-	v3_scale(&temp,&dist_ik,df_c_ik*bracket1);
+	/* part 2 of db_ij */
+	v3_scale(&temp,&dist_ik,df_c_ik*g);
 
-	/* sum up and scale ... */
+	/* sum up and add to db_ij */
 	v3_add(&temp,&temp,&force);
-	scale=bracket2_n_1*n*betan*(1+betan*bracket3_pow_1)*chi*(1.0/(2.0*n));
-	v3_scale(&temp,&temp,scale);
-
-	/* now construct an energy and a force out of that */
-	v3_scale(&temp,&temp,f_a);
-	v3_scale(&force,&dist_ij,df_a*b_ij);
-	v3_add(&temp,&temp,&force);
-	v3_scale(&temp,&temp,f_c);
-	v3_scale(&force,&dist_ij,df_c*b_ij*f_a);
-	v3_add(&force,&force,&temp);
-
-	/* add forces */
-	v3_add(&(ai->f),&(ai->f),&force);
-	/* energy is 0.5 f_r f_c, but we will sum it up twice ... */
-	moldyn->energy+=(0.25*f_a*b_ij*f_c);
+	v3_add(&(exchange->db_ij),&(exchange->db_ij),&temp);
 				
 	return 0;
 }