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Lennard-Jones in Fortran//F2PY
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C THE LENNARD-JONES POTENTIAL, GRADIENT AND THE VIRIAL | |
subroutine LennardJones(U,box,Epot,F,Vir,N) | |
cf2py intent(in) U | |
cf2py intent(in) box | |
cf2py intent(out) Epot | |
cf2py intent(out) F | |
cf2py intent(out) Vir | |
implicit none | |
double precision U,Epot,F,box,Vir | |
integer N | |
dimension U(3,N),F(3,N),box(3) | |
C | |
double precision zero,one,X,Y,Z,r2,r2i,r6i,feight,half, | |
* ftmp,four,rc2,rc2i,rc6i,ecut | |
integer i,j | |
C | |
parameter(zero=0.0D+00,one=1.0D+00,feight=48.0D+00, | |
* half=0.5D+00,four=4.0D+00,rc2=(6.25D+00), | |
* rc2i=one/rc2,rc6i=rc2i*rc2i*rc2i,ecut=rc6i*(rc6i-one)) | |
Epot = zero | |
Vir = zero | |
do i=1,N | |
F(1,i) = zero | |
F(2,i) = zero | |
F(3,i) = zero | |
enddo | |
do i=1,N-1 | |
do j=i+1,N | |
X = U(1,j) - U(1,i) | |
Y = U(2,j) - U(2,i) | |
Z = U(3,j) - U(3,i) | |
X = X - box(1)*nint(X/box(1)) | |
Y = Y - box(2)*nint(Y/box(2)) | |
Z = Z - box(3)*nint(Z/box(3)) | |
r2 = X*X + Y*Y + Z*Z | |
if(r2.lt.rc2) then | |
r2i = one / r2 | |
r6i = r2i*r2i*r2i | |
Epot = Epot + r6i*(r6i-one) - ecut | |
C WE MULTIPLY BY 48 HERE AND NOT LAST. | |
ftmp = feight*r6i*(r6i-half) | |
F(1,i) = F(1,i) - ftmp*X*r2i | |
F(2,i) = F(2,i) - ftmp*Y*r2i | |
F(3,i) = F(3,i) - ftmp*Z*r2i | |
F(1,j) = F(1,j) + ftmp*X*r2i | |
F(2,j) = F(2,j) + ftmp*Y*r2i | |
F(3,j) = F(3,j) + ftmp*Z*r2i | |
Vir = Vir + ftmp | |
endif | |
enddo | |
enddo | |
Epot = Epot * four | |
return | |
end |
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