Multilevel approach to the initial guess for self-consistent field calculations

A new multilevel approach is presented to the initial guess for self-consistent field (SCF) calculations, which combines the superposition of atomic densities (SAD) procedure and the density matrix of a semi-empirical quantum mechanics (SQM) calculation through projection. The proposed initial guess method produces a polarized, spin-specific initial density, while its computational costs are a few orders of magnitude lower than the expenses of a scheme that projects the density matrix of a standard quantum chemical calculation utilizing a minimal AO basis set (pMIN). The projected SQM density-based (pSQM) technique is thoroughly tested using the GFN2-xTB approach, and its efficiency and reliability are compared with those of the standard SAD and density matrix projection techniques. The results indicate that the calculations using the pSQM scheme require somewhat fewer SCF iteration steps compared with the SAD method, however, the SAD, the pSQM, and the pMIN techniques have similar performance.