The \(MnO_6\) system

The \(MnO_6\) compound is good example of the oxide crystal structure.

In the following we show how one can get converged molecular spinors (or molecular orbitals) of its ground state using single determinant SCF method.

We take into account two parameters for controling the SCF convergence - the .OPENFACTOR and the .OLEVEL keywords.

Here the noAcAc abbreviation (“no active-active”) means value of OPENFAC=0, what is no active-active interation among open-shell spinors. The wAcAc (“with active-active”) means OPENFAC=1, that is full active-active interation (or full open-shell contribution to the Fock matrix).

For simplicity, we employ the two-component Hamiltonian, see the keyword .X2C.

The way to the convergence

With the aim to achieve the SCF convergence we investigated these three ways:

1. Starting from noAcAc we get the SCF convergence. We save the DFCOEF file with molecular spinors. Input files to download are MnO6.x2c.bare_noacac.inp and MnO6.crystal.mol:

pam --inp=MnO6.x2c.bare_noacac.inp  --mol=MnO6.crystal.mol --get "DFCOEF=DFCOEF.MnO6.noAcAc"

2. Using AcAc with molecular spinors (the DFCOEF file) from the previous SCF run the \(MnO_6\) system does not converge with the default DIIS accelerator. The new input file to download is MnO6.x2c.wacac.inp:

pam --inp=MnO6.x2c.wacac.inp  --mol=MnO6.crystal.mol --put "DFCOEF.MnO6.noAcAc=DFCOEF"

3. Using the AcAc interaction with the parameter of OLEVEL=0.2, and, with the DFCOEF file from the point 1, we finally obtain the convergence. The total SCF energy of the system is -1617.5839185788545 a.u. (see the corresponding output file, MnO6.x2c.wacac_olvlshift_MnO6.crystal.out) The new DIRAC input file to download is MnO6.x2c.wacac.inp:

pam --inp=MnO6.x2c.wacac_olvlshift.inp  --mol=MnO6.crystal.mol --put "DFCOEF.MnO6.noAcAc=DFCOEF" --get "DFCOEF=DFCOEF.MnO6.wAcAc"

This tutorial shows that it is important to try all possible means to achieve the SCF convergence if the default setting is not working.