ANO-RCC basis sets

It is not possible to use normal AO basis sets in relativistic calculations The relativistic contraction of the inner shells makes it necessary to design new basis sets to account for this effect. Specially designed basis sets have therefore been constructed using the DKH Flamiltonian. These basis sets are of the atomic natural orbital (ANO) type and are constructed such that semi-core electrons can also be correlated. They have been given the name ANO-RCC (relativistic with core correlation) and cover all atoms of the Periodic Table.36-38 They have been used in most applications presented in this review. ANO-RCC are all-electron basis sets. Deep core orbitals are described by a minimal basis set and are kept frozen in the wave function calculations. The extra cost compared with using effective core potentials (ECPs) is therefore limited. ECPs, however, have been used in some studies, and more details will be given in connection with the specific application. The ANO-RCC basis sets can be downloaded from the home page of the MOLCAS quantum chemistry software (http //www.teokem.lu.se/molcas). 

Table 1. ANO-RCC basis sets used for the transition metal atoms...

We present the results for all atoms in Table 2. Three sets of results are given CCSD, CCSD(T), and CASPT2. For the first row atoms we have also performed calculations with two h-type basis functions added to the ANO-RCC basis set. Table 3 gives the root mean square (RMS) errors and also the maximum absolute error. 

B. O. Roos, The ANO-RCC basis sets are included in the MOLCAS-6 basis set library. They can be downloaded from the MOLCAS homepage (www.teokem.lu.se/molcas), 2004. 

The scalar part of the DKH Hamiltonian is added to the one-electron integrals. All scalar relativistic effects are then automatically included at all levels of theory. Such calculations have to be performed using basis sets that are adapted to such a Hamiltonian because of the contraction of the core orbitals. A new set of ANO-type basis sets have, therefore, been constructed, called ANO-RCC. They are available for aU atoms of the periodic table [30,63]. 

As an example of such a study we pick a small molecule PbF. Suppose we want to compute the potentials for the lower electronic states of this molecule, with the relativistic CASSCF/CASPT2 approach, how do we proceed Well, it should not be difficult. Relativistic basis sets (ANO-RCC) are available for Pb and F [30] and we choose a reasonably extended set Pb 25s22pl6dl2f4g/9s8p6d4f3g and F 14s9p4d3f2g/ 5s4p3d2flg. It is of quadruple zeta accuracy. 

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