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subroutine variational_montecarlo(a,dt,nmax,energy)
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2021-01-13 18:11:13 +01:00
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implicit none
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double precision, intent(in) :: a, dt
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integer*8 , intent(in) :: nmax
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double precision, intent(out) :: energy
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integer*8 :: istep
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double precision :: norm, r_old(3), r_new(3), d_old(3), sq_dt, chi(3)
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double precision, external :: e_loc
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2021-01-29 13:23:00 +01:00
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sq_dt = dsqrt(dt)
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! Initialization
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energy = 0.d0
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norm = 0.d0
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call random_gauss(r_old,3)
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do istep = 1,nmax
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call drift(a,r_old,d_old)
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call random_gauss(chi,3)
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r_new(:) = r_old(:) + dt * d_old(:) + chi(:)*sq_dt
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norm = norm + 1.d0
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energy = energy + e_loc(a,r_new)
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r_old(:) = r_new(:)
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end do
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energy = energy / norm
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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 :: dt = 0.2
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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)
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double precision :: ave, err
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do irun=1,nruns
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call variational_montecarlo(a,dt,nmax,X(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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end program qmc
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