eigen1d.f90

Ground state of the time-independent Gross-Pitaevskii equation

!***
! The following code is for use with the LAPACK library
!***

subroutine eigen(n,H,eigenval,eigenvec)
  !
  ! Return the eigenvector of H with the smallest eigenvalue
  ! H assumed symmetric
  !
  implicit none
  integer, parameter :: dp=kind(1.d0)

  integer, intent(in) :: n
  real(dp), dimension(n,n), intent(in) :: H
  real(dp), intent(out) :: eigenval
  real(dp), dimension(n), intent(out) :: eigenvec

  integer :: lwork,info
  real(dp), dimension(n) :: eval
  real(dp), dimension(n,n) :: evec
  real(dp), dimension(3*n) :: work
  character(len=1) :: jobz = 'V', uplo = 'U'

  lwork = 3*n
  evec = H

  call dsyev(jobz,uplo,n,evec,n,eval,work,lwork,info)

  if(info /= 0) then
     write(*,*) "Error with dsyev in subroutine eigen"
     stop
  end if

  eigenval = eval(1)
  eigenvec = evec(:,1)

  return
end subroutine eigen