Dynamic correlation energy of a molecular system can be calculated using the CASPT2 program module in MOLCAS. A CASPT2 calculation gives a second order perturbation estimate of the full CI energy using the CASSCF wave function of the system. As can be seen in the flowchart, CASPT2 follows a RASSCF calculation but in this case the RASSCF calculation must produce a CASSCF wave function. A new feature of the 5.0 version is the inclusion of the Multi-State CASPT2methods which allows thecoupling of different CASPT2 states provided they are included in a previos State-Average CASSCF calculation.
A sample input is given in Figure 3.6. The FROZen keyword specifies the number of orbitals of each symmetry which will not be included in the correlation. We have chosen the RASSCF INACtive orbitals to be frozen for this calculation. The remaining two keywords, CONVergence and MAXIter, are included with there default values. Keyword LROOt used in previous MOLCAS versions has been substituted by MULTistate. A single line follows indicating the number of simultaneously treated CASPT2 roots and the number of the roots in the previous SA-CASSCF calculation. Keyword MIXCi creates a new binary file containing the Perturbatively-Modify (PM) CASSCF wave function.
The CASPT2 section of the calculation output is more complicated than most other sections. In section 4.3.1 the meaning and significance of most of the features used and printed by the CASPT2 program are explained in the context of an actual example. We suggest the careful reading of that section to the full understanding of the results.
Figure 3.6. Sample input requesting the CASPT2 module to
calculate the CASPT2 energy
of a water molecule in C
symmetry with one frozen orbital.
&CASPT2 &END Frozen 1 0 0 0 Convergence 1.0e-07 Multistate 1 1 MaxIter 40 End Of Input
Broadly the output of the CASPT2 program begins with the title
from the input as well as the title from the SEWARD input.
It also contains the cartesian coordinates of the molecule and the
CASSCF wave function and orbital specifications. This is followed by
details about the type of Fock and H
operator used and, eventually,
the value of the level-shift parameter employed. It is possible then
to obtain, by input specifications, the quasi-canonical orbitals in
which the wave function will be represented. The following CI vector
and occupation number analysis will be performed using the
quasi-canonical orbitals.
Unlike previous versions, by default the option MAXIter is set to
ten and the program CASPT2 will solve the equations in the non-diagonal
solution to H, that is using the iterative solution of the non-linear
equations leading to the full approximation to H. If MAXIter
is set to zero CASPT2 will provide with the diagonal approximation
to H. Then, the reference CASSCF energy, the total CASPT2 energy, and the
weight of the reference function to the first-order wave function are printed.
In section 4.3.1 the importance of this last parameter is evaluated.
Two important sections follow. First a detailed report on small energy denominators, large components, and large energy contributions which will inform about the reliability of the calculation (see section 4.3.1) and finally the CASPT2 property section including the natural orbitals obtained as defined in the output and a number of approximated molecular properties.
If the Multistate option is used, the program will perform one CASPT2 calculation for each one of the selected roots, and finally the complete effective Hamiltonian containing the selected states will be solved to obtain the final MS-CASPT2 energies and PM-CASSCF wave functions [2].
The ORDINT file from the SEWARD module and the JOBIPH file from the RASSCF module are the required input files for the CASPT2 module. The orbitals are saved in the PT2ORB file. The new PM-CASSCF wave function generated in a MS-CASPT2 calculation is saved in the JOBMIX file.