fci

CIPSI algorithm in the full configuration interaction space.

The user point of view

• fci performs CIPSI calculations using a stochastic scheme for both the selection and the PT2 contribution,

• pt2 computes the PT2 contribution using the wave function stored in the EZFIO database.

The main keywords/options for this module are:

• determinants n_det_max : maximum number of Slater determinants in the CIPSI wave function. The fci program will stop when the size of the CIPSI wave function will exceed determinants n_det_max.

• perturbation pt2_max : absolute value of the PT2 to stop the CIPSI calculation. Once the abs(PT2) \(<\) perturbation pt2_max, the CIPSI calculation stops.

• determinants n_states : number of states to consider in the CIPSI calculation.

• determinants read_wf : if false, starts with a ROHF-like determinant, if true, starts with the current wave function(s) stored in the EZFIO directory.

Note

For a multi-state calculation, it is recommended to start with cis or cisd wave functions as a guess.

• determinants expected_s2 : expected value of \(\widehat{S^2}\) for the desired spin multiplicity.

• determinants s2_eig : if true, systematically add all the determinants needed to have a pure value of \(\widehat{S^2}\). Also, if true, it tracks only the states having the good determinants expected_s2.

The programmer’s point of view

This module was created with the cipsi module.

See also

The documentation of the cipsi module.

EZFIO parameters

energy

Calculated Selected FCI energy

energy_pt2

Calculated FCI energy + PT2

Programs

• fci

• pt2

Subroutines / functions

write_c_ij_ab:

File : write_c_ij_ab.irp.f

subroutine write_c_ij_ab

Needs:

read_wf

Calls:

routine()

Touches:

read_wf