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94 lines
1.8 KiB
Fortran
94 lines
1.8 KiB
Fortran
subroutine C16_lda_correlation_energy(nGrid,weight,rho,Ec)
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! Compute unrestricted Chachiyo's LDA correlation 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,nspin)
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! Local variables
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integer :: iG
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double precision :: ra,rb,r,rs
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double precision :: a_p,b_p,ec_p
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double precision :: a_f,b_f,ec_f
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double precision :: z,fz,ec_z
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! Output variables
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double precision :: Ec(nsp)
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! Coefficients for Chachiyo's LDA correlation
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a_p = (log(2d0) - 1d0)/(2d0*pi**2)
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b_p = 20.4562557d0
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a_f = (log(2d0) - 1d0)/(4d0*pi**2)
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b_f = 27.4203609d0
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! Compute LDA correlation energy
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Ec(:) = 0d0
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do iG=1,nGrid
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! Spin-up and spin-down densities
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ra = max(0d0,rho(iG,1))
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rb = max(0d0,rho(iG,2))
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! Total density
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r = ra + rb
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! Spin-up part contribution
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if(ra > threshold) then
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rs = (4d0*pi*ra/3d0)**(-1d0/3d0)
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ec_f = a_f*log(1d0 + b_f/rs + b_f/rs**2)
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Ec(1) = Ec(1) + weight(iG)*ec_f*ra
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endif
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! Opposite-spin contribution
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if(r > threshold) then
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rs = (4d0*pi*r/3d0)**(-1d0/3d0)
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ec_p = a_p*log(1d0 + b_p/rs + b_p/rs**2)
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ec_f = a_f*log(1d0 + b_f/rs + b_f/rs**2)
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z = (ra-rb)/r
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fz = (1d0 + z)**(4d0/3d0) + (1d0 - z)**(4d0/3d0) - 2d0
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fz = fz/(2d0*(2d0**(1d0/3d0) - 1d0))
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ec_z = ec_p + (ec_f - ec_p)*fz
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Ec(2) = Ec(2) + weight(iG)*ec_z*r
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endif
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! Spin-down contribution
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if(rb > threshold) then
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rs = (4d0*pi*rb/3d0)**(-1d0/3d0)
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ec_f = a_f*log(1d0 + b_f/rs + b_f/rs**2)
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Ec(3) = Ec(3) + weight(iG)*ec_f*rb
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endif
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enddo
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Ec(2) = Ec(2) - Ec(1) - Ec(3)
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end subroutine C16_lda_correlation_energy
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