Active-space calculations

In active-space calculations, the total orbital space is usually partitioned into external core orbitals (c), active orbitals (a), and unoccupied virtual (external) orbitals (v). (There can additionally be some frozen core orbitals that remain doubly occupied throughout the calculation.)... 

CASSCF see Complete active space calculation Catalase... 

Multireference character, complete active space calculation, 12, 29 Multivariate analysis analytical methods, 624 SIMPLISMA program, 624, 702 Mutations, DNA oxidative damage, 616 Myeloperoxidase, tyrosyl free radicals, 610 Myocardial ischemia, reactive oxygen species in blood, 612... 

Numbers without parentheses from 6ai, 3b2, 3bi, la2 active space calculation Numbers inside parentheses from B 4ai, 2b2, 2bi active space. 

H. A Direct Configuration-interaction Method for Complete Active Space Calculations... 

Figure 1. Total nonrelativistic multi-configuration Hartree-Fock energy, relativistic corrections (estimated as the difference between the multi-configuration Dirac-Hartree-Fock and Hartree-Fock energies) and correlation contributions (estimated from correlation energy density functional calculations) for the group 4 elements. The multi-configuration treatments were carried out with the atomic structure code GRASP [78] and correspond to complete active space calculations with the open valence p shell as active space. The nonrelativistic results were obtained by multiplying the velocity of light with a factor of 10 .

Figure 1. Benzene E u transition as a function of active space calculated with Cl Singles and RPA methods combined with INDO/S model. Adapted from Baker and Zerner ref 139.

Roos B O 1987 The complete active space self-consistent field method and its applications in electronic structure calculations Adv. Chem. Phys. 69 399-445... 

Unfortunately, these methods require more technical sophistication on the part of the user. This is because there is no completely automated way to choose which configurations are in the calculation (called the active space). The user must determine which molecular orbitals to use. In choosing which orbitals to include, the user should ensure that the bonding and corresponding antibonding orbitals are correlated. The orbitals that will yield the most correlation... 

An MCSCF calculation in which all combinations of the active space orbitals are included is called a complete active space self-consistent held (CASSCF) calculation. This type of calculation is popular because it gives the maximum correlation in the valence region. The smallest MCSCF calculations are two-conhguration SCF (TCSCF) calculations. The generalized valence bond (GVB) method is a small MCSCF including a pair of orbitals for each molecular bond. 

A CASSCF calculation is a combination of an SCF computation with a full Configuration Interaction calculation involving a subset of the orbitals. The orbitals involved in the Cl are known as the active space. In this way, the CASSCF method optimizes the orbitals appropriately for the excited state. In contrast, the Cl-Singles method uses SCF orbitals for the excited state. Since Hartree-Fock orbitals are biased toward the ground state, a CASSCF description of the excited state electronic configuration is often an improvement. 

A CASSCF calculation is requested in Gaussian with the CASSCF keyword, which requires two integer arguments the number of electrons and the number of orbitals in the active space. The active space is defined assuming that the electrons come from as many of the highest occupied molecular orbitals as are needed to obtain the specified number of electrons any remaining required orbitals are taken from the lowest virtual orbitals. 

Accordingly, we will swap orbitals 6 and 13. We define the active space to comprise these orbitals by using the Gues =Alter keyword in the route section for the first job step in our second calculation series ... 

Perform a series of state-averaged 4,4 CAS calculations on the ground state, using the active space you have selected. Normally, a CAS calculation optimizes the orbitals and hence the wavefimction for the state of interest (as specified by the NRoot option). However, in a state-averaged CAS, orbitals are optimi. ed to provide the best description of the specified group of states as a whole that is possible with a single set of orbitals... 

In this section, we describe calculations of the P,T-odd interaction constant Wd for the ground (X2E, 2) states of YbF and BaF molecules using all-electron DF orbitals and a restricted active space (RAS) configuration interaction (Cl) treatment. 

The active space used for both systems in these calculations is sufficiently large to incorporate important core-core, core-valence, and valence-valence electron correlation, and hence should be capable of providing a reliable estimate of Wj- In addition to the P,T-odd interaction constant Wd, we also compute ground to excited state transition energies, the ionization potential, dipole moment (pe), ground state equilibrium bond length and vibrational frequency (ov) for the YbF and pe for the BaF molecule. 

The P,T-odd constant Wd and dipole moment p(. for the ground state of the BaF molecule are calculated using the restricted active space (RAS) configuration... 

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