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59 lines
1.6 KiB
Fortran
59 lines
1.6 KiB
Fortran
subroutine metropolis_montecarlo(a,nmax,tau,energy,accep)
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implicit none
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double precision, intent(in) :: a
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integer*8 , intent(in) :: nmax
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double precision, intent(in) :: tau
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double precision, intent(out) :: energy
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double precision, intent(out) :: accep
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integer*8 :: istep
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double precision :: r_old(3), r_new(3), psi_old, psi_new
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double precision :: v, ratio
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integer*8 :: n_accep
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double precision, external :: e_loc, psi, gaussian
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energy = 0.d0
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n_accep = 0_8
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call random_number(r_old)
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r_old(:) = tau * (2.d0*r_old(:) - 1.d0)
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psi_old = psi(a,r_old)
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do istep = 1,nmax
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call random_number(r_new)
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r_new(:) = r_old(:) + tau * (2.d0*r_new(:) - 1.d0)
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psi_new = psi(a,r_new)
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ratio = (psi_new / psi_old)**2
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call random_number(v)
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if (v <= ratio) then
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r_old(:) = r_new(:)
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psi_old = psi_new
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n_accep = n_accep + 1_8
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endif
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energy = energy + e_loc(a,r_old)
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end do
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energy = energy / dble(nmax)
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accep = dble(n_accep) / dble(nmax)
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end subroutine metropolis_montecarlo
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program qmc
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implicit none
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double precision, parameter :: a = 0.9d0
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double precision, parameter :: tau = 1.3d0
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integer*8 , parameter :: nmax = 100000
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integer , parameter :: nruns = 30
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integer :: irun
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double precision :: X(nruns), Y(nruns)
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double precision :: ave, err
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do irun=1,nruns
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call metropolis_montecarlo(a,nmax,tau,X(irun),Y(irun))
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enddo
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call ave_error(X,nruns,ave,err)
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print *, 'E = ', ave, '+/-', err
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call ave_error(Y,nruns,ave,err)
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print *, 'A = ', ave, '+/-', err
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end program qmc
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