Z_internal.f90

SUBROUTINE partition(T, nu, n, label, F, Fmax, ele, symm, Trot, Zvib, Zrot, Zint)

USE thermo_constants

! This subroutine computes molecular partition functions at given
! thermodynamical conditions:
!
! Args:
!       T (real)            : temperature
!       nu (real, array)    : frequencies
!       n (integer)         : number of hydrogenations
!       label (integer)     : number identyfing the specific hydrogenated specie for which we want to compute the partition function
!       F (integer)         : degrees of freedom
!       Fmax  (integer)     : max degrees of freedom
!       ele (integer, array): alpha/beta electrons
!       symm (integer, array)       : symmetry numbers
!       Trot (real, array)  : rotational temperatures
!    
! Out:
!       Zvib, Zrot, Zint (real) : vibrational, rotational and internal partition function 

IMPLICIT NONE

! Computing internal partition function

! Declaration of input variables
INTEGER(4) :: n, label, F, Fmax
REAL(8) :: T              
REAL(8), DIMENSION(0:n,Fmax) :: nu 
REAL(8), DIMENSION(0:n,2) :: ele
REAL(8), DIMENSION(0:n) :: symm
REAL(8), DIMENSION(0:n,3) :: Trot

! Declaration of side and output variables
INTEGER(4) :: Nele
REAL(8) :: theta, expo
REAL(8) :: Zvib, Zele, Zrot
REAL(8) :: Zint

! Declaration of counters
INTEGER :: i

! Computing Vibrational Partition function
Zvib = 1.0d0
DO i = 1,F
        expo = (-h*c*nu(label,i)*100.0d0)/(kb*T )
        Zvib = Zvib*( 1.0d0/(1.0d0-exp(expo) ) )
END DO

! Computing Rotational Partition Function
theta = ( Trot(label,1)*Trot(label,2)*Trot(label,3) )**onethird
Zrot = (1.0d0/symm(label))*((T/theta)**threesecond)*sqrt(pi)
! Computing Electronical Partition Function
Nele = ele(label,1)+ele(label,2)
Zele = 1 + mod(Nele,2)
! Computing Total Partition Function
Zint = Zvib*Zrot*Zele

END SUBROUTINE