How To Perform a Calculation

Single point calculations:

Call the ricc2 program after a converged SCF calculation, which can be carried either with the dscf or the ridft program.

Geometry optimizations and molecular dynamics:

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 and vibrational frequencies:

Force constants can be calculated by numerical differentiation of the gradients. Invoke for this NumForce with the -level CC2 option; see Chapter 11 for details about Numforce. The usage of the Numforce interface for excited states is restricted to C₁ 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 9.3.1) in datagroup $ricc2 (see Section 15.2.13). 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 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 a reference wavefunction which employed only the RI-J approximation for the Coulomb matrix but 4-index integrals for the exchange matrix is presently not supported. The implementation gradients in ricc2 assumes that the reference wavefunction has either been calculated with RI 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 15.2.5 for 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.