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https://github.com/pfloos/quack
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51 lines
1.0 KiB
Fortran
51 lines
1.0 KiB
Fortran
subroutine B88_gga_exchange_energy(nGrid,weight,rho,drho,Ex)
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! Compute 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(3,nGrid)
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! Local variables
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integer :: iG
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double precision :: alpha,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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alpha = -(3d0/2d0)*(3d0/(4d0*pi))**(1d0/3d0)
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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,rho(iG))
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if(r > threshold) then
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g = 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)*alpha*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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end subroutine B88_gga_exchange_energy
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