Call the ricc2 program after a converged SCF calculation, which can be carried
either with the dscf or the ridft program.
Invoke jobex with the -level cc2 option; see Section 5.1 for additional
options and parameters of the jobex script that might be needed or useful for
geometry optimizations and ab initio molecular dynamics calculations.
Force constants can be calculated by numerical differentiation of the gradients.
Invoke for this NumForce with the -level cc2 option; see Chapter 14 for
details about Numforce. The usage of the Numforce interface for excited states
is restricted to C_{1} symmetry.
Note: using ricc2 in connection with jobex or Numforce requires that the method and the electronic state, for which the gradient should be calculated and written to the interface files, is specified in the option geoopt (see Section 10.3.1) in datagroup $ricc2 (see Section 20.2.19). For calculations on excited states this state has in addition to be included in the input for excitation energies in datagroup $excitations.
RI-SCF reference wavefunctions: The ricc2 program can be used in combination with conventional SCF or with the RI-J and RI-JK approximations for SCF, with the exception that the calculation of gradients for reference wavefunctions which employ only the RI-J approximation for the Coulomb matrix but 4-index integrals for the exchange matrix is presently not supported. The implementation of gradients in ricc2 assumes that the reference wavefunction has either been calculated without RI-J approximation (using dscf) or with the RI-JK approximation (using ridft).
See Chapter 6 for a discussion of the RI approximations in SCF calculations and 20.2.7 for the required input. In geometry optimizations with jobex and for the calculation of force constants and vibrational spectra with NumForce, the ricc2 program is used in combination with the RI-JK approximation for the Hatree-Fock calculation (using ridft) if jobex and NumForce are invoked with the -rijk option.