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subroutine force_lj ( i, j, rcutsq, rijsq, sigma, epslon, v, & |
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fx, fy, fz, rxij, ryij, rzij, natoms ) |
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implicit none |
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! Passed parameters |
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integer :: natoms ! the number of atoms |
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integer :: i, j ! the index of the two atoms |
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double precision rcutsq ! the square of rcut (needed for the shift potential) |
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double precision rijsq ! the square of the distance twixt i and j |
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double precision v ! the potential energy |
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double precision rxij, ryij, rzij ! vector components of the distance |
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! Passed arrays |
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double precision, dimension(natoms) :: sigma ! the distance parameters |
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double precision, dimension(natoms) :: epslon ! the wel depth parameters |
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double precision, dimension(natoms) :: fx, fy, fz ! the force arrays |
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! local variables |
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double precision sr2, sr6, vij, vsij, fij |
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double precision sigm, sigsq, epsl |
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double precision fxij, fyij, fzij |
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!******************************************************* |
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epsl = dsqrt( epslon(i) * epslon(j) ) |
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sigm = ( sigma(i) + sigma(j) ) / 2.0d0 |
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sigsq = sigm * sigm |
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sr2 = sigsq / rijsq |
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sr6 = sr2 * sr2 * sr2 |
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vij = epsl * sr6 * ( sr6 - 1.0d0 ) |
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fij = epsl * sr6 * ( sr6 - 0.5d0 ) |
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fij = fij / rijsq |
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fxij = rxij * fij * 48.0d0 |
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fyij = ryij * fij * 48.0d0 |
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fzij = rzij * fij * 48.0d0 |
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fx(i) = fx(i) - fxij |
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fy(i) = fy(i) - fyij |
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fz(i) = fz(i) - fzij |
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fx(j) = fx(j) + fxij |
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fy(j) = fy(j) + fyij |
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fz(j) = fz(j) + fzij |
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! calculate the shifted potential |
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sr2 = sigsq / rcutsq |
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sr6 = sr2 * sr2 * sr2 |
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vsij = epsl * sr6 * ( sr6 - 1.0d0 ) |
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! calculate the continous potential |
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v = v + 4.0d0 * ( vij - vsij ) |
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end subroutine force_lj |
