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qmc-lttc/qmc_metropolis.f90

72 lines
1.6 KiB
Fortran

subroutine metropolis_montecarlo(a,nmax,dt,energy,accep)
implicit none
double precision, intent(in) :: a
integer*8 , intent(in) :: nmax
double precision, intent(in) :: dt
double precision, intent(out) :: energy
double precision, intent(out) :: accep
double precision :: r_old(3), r_new(3), psi_old, psi_new
double precision :: v, ratio
integer*8 :: n_accep
integer*8 :: istep
double precision, external :: e_loc, psi, gaussian
energy = 0.d0
n_accep = 0_8
call random_number(r_old)
r_old(:) = dt * (2.d0*r_old(:) - 1.d0)
psi_old = psi(a,r_old)
do istep = 1,nmax
energy = energy + e_loc(a,r_old)
call random_number(r_new)
r_new(:) = r_old(:) + dt*(2.d0*r_new(:) - 1.d0)
psi_new = psi(a,r_new)
ratio = (psi_new / psi_old)**2
call random_number(v)
if (v <= ratio) then
n_accep = n_accep + 1_8
r_old(:) = r_new(:)
psi_old = psi_new
endif
end do
energy = energy / dble(nmax)
accep = dble(n_accep) / dble(nmax)
end subroutine metropolis_montecarlo
program qmc
implicit none
double precision, parameter :: a = 1.2d0
double precision, parameter :: dt = 1.0d0
integer*8 , parameter :: nmax = 100000
integer , parameter :: nruns = 30
integer :: irun
double precision :: X(nruns), Y(nruns)
double precision :: ave, err
do irun=1,nruns
call metropolis_montecarlo(a,nmax,dt,X(irun),Y(irun))
enddo
call ave_error(X,nruns,ave,err)
print *, 'E = ', ave, '+/-', err
call ave_error(Y,nruns,ave,err)
print *, 'A = ', ave, '+/-', err
end program qmc