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- Incorrect/inconsistent results from zgeev() LAPACK 2 answers
I stumbled upon an odd behavior when using the lapack routine zheev(). There are two issues which I do not understand
1) One of my global variables seems to be overwritten by zheev(). The following small program shows it:
[compiled with gfortran -o test test.f90 -llapack -lblas]
program test
implicit none
integer, parameter :: dp = 8
integer, parameter :: dim = 3
integer :: l
real(dp), parameter :: kmin = -0.03_dp
real(dp), parameter :: kmax = 0.03_dp
integer, parameter :: steps = 100
real(dp) :: stepD = (kmax - kmin)/real(steps)
complex(dp) :: matrix(dim,dim)=0.
integer :: info
integer :: rwork=3*dim-2, lwork, lwmax=100
real(dp) :: evals(dim)
complex(dp) :: work(3*dim-2)
lwork=-1
call zheev('N','U',size(matrix,1), matrix, dim, evals, work, lwork, &
rwork, info)
lwork = min( lwmax, int( work(1) ))
do l = 0, 3
write(*,*) stepD
call zheev('N', 'U', size(matrix,1), matrix, dim, evals, work, &
lwork, rwork, info)
write(*,*) stepD
end do
end program test
The output is
5.9999999999999995E-004
0.0000000000000000
0.0000000000000000
0.0000000000000000
0.0000000000000000
0.0000000000000000
0.0000000000000000
0.0000000000000000
This can be cured by setting stepD to a parameter. But I do not understand this behavior. It is getting even more weird:
2) Setting lwork in the definition to some value like
integer :: rwork=3*dim-2, lwork=10, lwmax=100
(simply change the corresponding line above) gives the following result:
5.9999999999999995E-004
5.9999991208314896E-004
5.9999991208314896E-004
5.9999991208314896E-004
5.9999991208314896E-004
5.9999991208314896E-004
5.9999991208314896E-004
5.9999991208314896E-004
How can this happen? Setting lwork to 10 should have no effect since later on it is set to -1. An important notion is: If one removes
lwork=-1
call zheev('N','U',size(matrix,1), matrix, dim, evals, work, lwork, &
rwork, info)
lwork = min( lwmax, int( work(1) ))
the code works fine.
