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49 lines
960 B
Fortran
49 lines
960 B
Fortran
subroutine UB88_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 :: b
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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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b = 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)*r**(4d0/3d0)*(CxLSDA - b*x**2/(1d0 + 6d0*b*x*asinh(x)))
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end if
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end do
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end subroutine UB88_gga_exchange_energy
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