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https://github.com/pfloos/quack
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52 lines
1.0 KiB
Fortran
52 lines
1.0 KiB
Fortran
subroutine RB88_gga_exchange_energy(nGrid,weight,rho,drho,Ex)
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! Compute restricted version of Becke's 88 GGA exchange energy
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implicit none
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include 'parameters.h'
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! Input variables
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integer,intent(in) :: nGrid
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double precision,intent(in) :: weight(nGrid)
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double precision,intent(in) :: rho(nGrid)
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double precision,intent(in) :: drho(ncart,nGrid)
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! Local variables
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integer :: iG
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double precision :: beta
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double precision :: r,g,x
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! Output variables
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double precision :: Ex
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! Coefficients for B88 GGA exchange functional
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beta = 0.0042d0
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! Compute GGA exchange energy
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Ex = 0d0
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do iG=1,nGrid
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r = max(0d0,0.5d0*rho(iG))
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if(r > threshold) then
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g = 0.25d0*(drho(1,iG)**2 + drho(2,iG)**2 + drho(3,iG)**2)
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x = sqrt(g)/r**(4d0/3d0)
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Ex = Ex + weight(iG)*CxLDA*r**(4d0/3d0) &
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- weight(iG)*beta*x**2*r**(4d0/3d0)/(1d0 + 6d0*beta*x*asinh(x))
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end if
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end do
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Ex = 2d0*Ex
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end subroutine RB88_gga_exchange_energy
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