1 |
mmeineke |
10 |
|
2 |
|
|
subroutine force_lj ( i, j, rcutsq, rijsq, sigma, epslon, v, & |
3 |
|
|
fx, fy, fz, rxij, ryij, rzij, natoms ) |
4 |
|
|
implicit none |
5 |
|
|
|
6 |
|
|
|
7 |
|
|
! Passed parameters |
8 |
|
|
|
9 |
|
|
integer :: natoms ! the number of atoms |
10 |
|
|
integer :: i, j ! the index of the two atoms |
11 |
|
|
|
12 |
|
|
double precision rcutsq ! the square of rcut (needed for the shift potential) |
13 |
|
|
double precision rijsq ! the square of the distance twixt i and j |
14 |
|
|
double precision v ! the potential energy |
15 |
|
|
double precision rxij, ryij, rzij ! vector components of the distance |
16 |
|
|
|
17 |
|
|
! Passed arrays |
18 |
|
|
|
19 |
|
|
double precision, dimension(natoms) :: sigma ! the distance parameters |
20 |
|
|
double precision, dimension(natoms) :: epslon ! the wel depth parameters |
21 |
|
|
double precision, dimension(natoms) :: fx, fy, fz ! the force arrays |
22 |
|
|
|
23 |
|
|
! local variables |
24 |
|
|
|
25 |
|
|
double precision sr2, sr6, vij, vsij, fij |
26 |
|
|
double precision sigm, sigsq, epsl |
27 |
|
|
double precision fxij, fyij, fzij |
28 |
|
|
|
29 |
|
|
!******************************************************* |
30 |
|
|
|
31 |
|
|
epsl = dsqrt( epslon(i) * epslon(j) ) |
32 |
|
|
sigm = ( sigma(i) + sigma(j) ) / 2.0d0 |
33 |
|
|
sigsq = sigm * sigm |
34 |
|
|
|
35 |
|
|
sr2 = sigsq / rijsq |
36 |
|
|
sr6 = sr2 * sr2 * sr2 |
37 |
|
|
|
38 |
|
|
vij = epsl * sr6 * ( sr6 - 1.0d0 ) |
39 |
|
|
|
40 |
|
|
fij = epsl * sr6 * ( sr6 - 0.5d0 ) |
41 |
|
|
fij = fij / rijsq |
42 |
|
|
|
43 |
|
|
fxij = rxij * fij * 48.0d0 |
44 |
|
|
fyij = ryij * fij * 48.0d0 |
45 |
|
|
fzij = rzij * fij * 48.0d0 |
46 |
|
|
|
47 |
|
|
fx(i) = fx(i) - fxij |
48 |
|
|
fy(i) = fy(i) - fyij |
49 |
|
|
fz(i) = fz(i) - fzij |
50 |
|
|
|
51 |
|
|
fx(j) = fx(j) + fxij |
52 |
|
|
fy(j) = fy(j) + fyij |
53 |
|
|
fz(j) = fz(j) + fzij |
54 |
|
|
|
55 |
|
|
! calculate the shifted potential |
56 |
|
|
|
57 |
|
|
sr2 = sigsq / rcutsq |
58 |
|
|
sr6 = sr2 * sr2 * sr2 |
59 |
|
|
|
60 |
|
|
vsij = epsl * sr6 * ( sr6 - 1.0d0 ) |
61 |
|
|
|
62 |
|
|
! calculate the continous potential |
63 |
|
|
|
64 |
|
|
v = v + 4.0d0 * ( vij - vsij ) |
65 |
|
|
|
66 |
|
|
end subroutine force_lj |