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qmc-lttc/vmc_metropolis.f90

96 lines
2.2 KiB
Fortran

subroutine variational_montecarlo(a,dt,nmax,energy,accep)
implicit none
double precision, intent(in) :: a, dt
integer*8 , intent(in) :: nmax
double precision, intent(out) :: energy, accep
integer*8 :: istep
integer*8 :: n_accep
double precision :: sq_dt, chi(3), d2_old, prod, u
double precision :: psi_old, psi_new, d2_new, argexpo, q
double precision :: r_old(3), r_new(3)
double precision :: d_old(3), d_new(3)
double precision, external :: e_loc, psi
sq_dt = dsqrt(dt)
! Initialization
energy = 0.d0
n_accep = 0_8
call random_gauss(r_old,3)
call drift(a,r_old,d_old)
d2_old = d_old(1)*d_old(1) + &
d_old(2)*d_old(2) + &
d_old(3)*d_old(3)
psi_old = psi(a,r_old)
do istep = 1,nmax
energy = energy + e_loc(a,r_old)
call random_gauss(chi,3)
r_new(:) = r_old(:) + dt*d_old(:) + chi(:)*sq_dt
call drift(a,r_new,d_new)
d2_new = d_new(1)*d_new(1) + &
d_new(2)*d_new(2) + &
d_new(3)*d_new(3)
psi_new = psi(a,r_new)
! Metropolis
prod = (d_new(1) + d_old(1))*(r_new(1) - r_old(1)) + &
(d_new(2) + d_old(2))*(r_new(2) - r_old(2)) + &
(d_new(3) + d_old(3))*(r_new(3) - r_old(3))
argexpo = 0.5d0 * (d2_new - d2_old)*dt + prod
q = psi_new / psi_old
q = dexp(-argexpo) * q*q
call random_number(u)
if (u <= q) then
n_accep = n_accep + 1_8
r_old(:) = r_new(:)
d_old(:) = d_new(:)
d2_old = d2_new
psi_old = psi_new
end if
end do
energy = energy / dble(nmax)
accep = dble(n_accep) / dble(nmax)
end subroutine variational_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), accep(nruns)
double precision :: ave, err
do irun=1,nruns
call variational_montecarlo(a,dt,nmax,X(irun),accep(irun))
enddo
call ave_error(X,nruns,ave,err)
print *, 'E = ', ave, '+/-', err
call ave_error(accep,nruns,ave,err)
print *, 'A = ', ave, '+/-', err
end program qmc