Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction

Area of Science:

• Quantum Chemistry
• Computational Physics

Background:

• Accurate calculation of molecular dipole properties is crucial for understanding chemical and physical phenomena.
• Higher-order dipole properties, such as hyperpolarizabilities, are challenging to compute accurately, especially for multireference systems.

Purpose of the Study:

• To develop and validate a controlled method for calculating higher-order dipole properties up to second hyperpolarizabilities.
• To assess the accuracy and applicability of the proposed method by comparing it with full configuration interaction (FCI) results.

Main Methods:

• Sum-over-states (SOS) calculations utilizing compact wavefunctions from Monte Carlo configuration interaction (MC-CI).
• Application to small molecules (HF, H4, CO, Ne) and molecular oxygen across various basis sets (6-31g, aug-cc-pVDZ, aug-cc-pVTZ, aug-cc-pVQZ).
• Comparison with FCI results for benchmark systems.

Main Results:

• The MC-CI SOS approach successfully reproduces accurate higher-order dipole properties, comparable to FCI.
• Frequency-dependent properties and behavior with increasing basis set size were investigated.
• The method demonstrated capability in handling multireference character and unusual electronic structures.

Conclusions:

• The proposed MC-CI SOS method offers a controlled and accurate pathway to compute higher-order dipole properties.
• This approach is effective for systems with varying degrees of multireference character and basis set dependency.
• The study validates the method's potential for advanced quantum chemical calculations.