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subroutine force_VDW ( 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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real(kind=8) :: rcutsq ! the square of rcut (needed for the shift potential) |
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real(kind=8) :: rijsq ! the square of the distance twixt i and j |
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real(kind=8) :: v ! the potential energy |
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real(kind=8) :: rxij, ryij, rzij ! vector components of the distance |
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! Passed arrays |
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real(kind=8), dimension(natoms) :: sigma ! the distance parameters |
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real(kind=8), dimension(natoms) :: epslon ! the wel depth parameters |
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real(kind=8), dimension(natoms) :: fx, fy, fz ! the force arrays |
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! local variables |
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real(kind=8), parameter :: beta = 2.25_8, alpha = 1.84e5_8 |
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real(kind=8) :: sr2, sr6, vij, vsij, fij |
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real(kind=8) :: srexp, sr, sr_i |
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real(kind=8) :: sigm, sigsq, epsl |
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real(kind=8) :: fxij, fyij, fzij |
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real(kind=8) :: preterm, r_exp_6 |
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! ********************************************************************* |
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epsl = sqrt( epslon(i) * epslon(j) ) |
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sigm = ( sigma(i) + sigma(j) ) / 2.0_8 |
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sigsq = sigm * sigm |
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sr2 = sigsq / rijsq |
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sr = sqrt(sr2) |
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sr_i = 1.0_8 / sr |
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sr6 = beta * sr2 * sr2 * sr2 |
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srexp = alpha * dexp( -12.0_8 * sr_i ) |
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vij = epsl * ( srexp - sr6 ) |
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r_exp_6= ( 2.0_8 * srexp ) - ( sr * sr6 ) |
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preterm= ( 6.0_8 * epsl ) / ( sigsq * sr_i ) |
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fij = preterm * r_exp_6 |
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fxij = rxij * fij |
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fyij = ryij * fij |
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fzij = rzij * fij |
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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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sr = sqrt( sr2 ) |
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sr_i = 1.0_8 / sr |
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sr6 = beta * sr2 * sr2 * sr2 |
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srexp = alpha * exp(-12.0_8 * sr_i) |
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vsij = epsl * ( srexp - sr6 ) |
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! calculate the continous potential |
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v = v + vij - vsij |
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end subroutine force_vdw |
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