2021-01-13 18:11:13 +01:00
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subroutine variational_montecarlo(a,tau,nmax,energy,accep_rate)
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implicit none
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double precision, intent(in) :: a, tau
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integer*8 , intent(in) :: nmax
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double precision, intent(out) :: energy, accep_rate
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integer*8 :: istep
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2021-01-26 13:11:57 +01:00
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double precision :: sq_tau, chi(3), d2_old, prod, u
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2021-01-13 18:11:13 +01:00
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double precision :: psi_old, psi_new, d2_new, argexpo, q
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double precision :: r_old(3), r_new(3)
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double precision :: d_old(3), d_new(3)
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double precision, external :: e_loc, psi
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sq_tau = dsqrt(tau)
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2021-01-20 21:18:22 +01:00
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2021-01-13 18:11:13 +01:00
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! Initialization
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energy = 0.d0
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accep_rate = 0.d0
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call random_gauss(r_old,3)
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call drift(a,r_old,d_old)
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d2_old = d_old(1)*d_old(1) + d_old(2)*d_old(2) + d_old(3)*d_old(3)
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psi_old = psi(a,r_old)
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do istep = 1,nmax
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call random_gauss(chi,3)
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r_new(:) = r_old(:) + tau * d_old(:) + chi(:)*sq_tau
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call drift(a,r_new,d_new)
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d2_new = d_new(1)*d_new(1) + d_new(2)*d_new(2) + d_new(3)*d_new(3)
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psi_new = psi(a,r_new)
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! Metropolis
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prod = (d_new(1) + d_old(1))*(r_new(1) - r_old(1)) + &
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2021-01-26 13:11:57 +01:00
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(d_new(2) + d_old(2))*(r_new(2) - r_old(2)) + &
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(d_new(3) + d_old(3))*(r_new(3) - r_old(3))
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2021-01-13 18:11:13 +01:00
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argexpo = 0.5d0 * (d2_new - d2_old)*tau + prod
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q = psi_new / psi_old
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q = dexp(-argexpo) * q*q
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call random_number(u)
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if (u<q) then
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accep_rate = accep_rate + 1.d0
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r_old(:) = r_new(:)
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d_old(:) = d_new(:)
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d2_old = d2_new
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psi_old = psi_new
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end if
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energy = energy + e_loc(a,r_old)
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end do
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2021-01-26 13:11:57 +01:00
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energy = energy / dble(nmax)
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accep_rate = dble(accep_rate) / dble(nmax)
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2021-01-13 18:11:13 +01:00
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end subroutine variational_montecarlo
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program qmc
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implicit none
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double precision, parameter :: a = 0.9
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double precision, parameter :: tau = 1.0
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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), accep(nruns)
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double precision :: ave, err
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do irun=1,nruns
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call variational_montecarlo(a,tau,nmax,X(irun),accep(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(accep,nruns,ave,err)
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print *, 'A = ', ave, '+/-', err
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end program qmc
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