# 3.3.20. FFPT — A Finite Field Perturbation Program¶

Many molecular properties of wave functions can be computed using the FFPT program module in Molcas. It adds the requested operator to the integrals computed by the SEWARD module. This must be done before the Molcas module calculating the required wave function is requested so the FFPT module is best run directly after the SEWARD module.

The TITLe keyword behaviors in a similar fashion to other Molcas modules. The sample input below contains the FFPT input requesting that the dipole moment operator be added to the integrals using the DIPOle keyword. The size and direction is specified using the COMP keyword which accepts free format input. We can compute the dipole of the molecule by numerical determination of the gradient of the energy curve determined for several values of the dipole operator. From the second derivative we can obtain the polarizability component.

Sample input requesting the FFPT module to include a dipole moment operator in the integral file:

&FFPT
Title= Finite Perturbation with a dipole in the x negative of strength 0.1 au
FFPT
Dipole
Comp
X -0.1


## 3.3.20.1. FFPT Output¶

The FFPT section of the output is short and self explanatory. The ONEINT file is updated with the requested operator.