Optimization of selected molecular orbitals in group basis sets

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate determination of electronic orbitals is crucial for understanding molecular properties.
• Traditional methods using large basis sets are computationally expensive.
• Developing efficient approximations is essential for scaling electronic structure calculations.

Purpose of the Study:

• To derive and validate a local basis equation for determining group orbitals.
• To minimize computational cost by using reduced, group-specific basis sets.
• To compare the accuracy of the local basis equation with existing methods like the Huzinaga equation.

Main Methods:

• Derivation of a local basis equation for group orbitals.
• Application of the local basis equation to molecular systems (HCl, PCl3, n-hexane).
• Comparison with Hartree-Fock-Roothaan (HFR) and Huzinaga equations using varying basis set sizes.

Main Results:

• The local basis equation accurately determines orbitals and minimizes energy with reduced basis sets.
• Calculated orbital energies for HCl were within 0.001 hartree of HFR values using a subset of basis functions.
• Total energy for HCl was only 0.003 hartree higher than HFR, significantly outperforming the Huzinaga equation approximation.
• Accurate reproduction of n-hexane energy and conformer energy differences was achieved.

Conclusions:

• The local basis equation offers a practical approach for accurate electronic structure calculations at reduced computational cost.
• It enables the use of predetermined fixed core and reduced valence orbitals.
• The method shows promise for linear scaling electronic structure calculations, especially for large systems.