Excited states using semistochastic heat-bath configuration interaction.

Area of Science:

• Quantum chemistry
• Computational physics
• Molecular spectroscopy

Background:

• Accurate calculation of molecular excited states is crucial for understanding chemical reactions and photophysics.
• Existing methods often face limitations in computational cost and accuracy for excited-state properties.

Purpose of the Study:

• To extend the heat-bath configuration interaction (HCI) and semistochastic perturbation theory algorithms for excited-state calculations.
• To improve the efficiency and accuracy of high-level quantum chemical computations.

Main Methods:

• Utilized a heat-bath configuration interaction (HCI) algorithm and a semistochastic algorithm for multireference perturbation theory.
• Employed time-reversal symmetry to significantly reduce memory requirements.
• Introduced an extrapolation technique to achieve energies extrapolated to the full configuration interaction (CI) limit.

Main Results:

• Successfully computed fourteen low-lying potential energy surfaces for the carbon dimer (C2) using the cc-pV5Z basis set.
• Achieved an estimated energy error of 30-50 μHa compared to full CI.
• Obtained excitation energies with a mean absolute deviation of 0.02 eV relative to experimental values.

Conclusions:

• The developed computational method provides highly accurate excited-state energies for molecular systems.
• The approach demonstrates significant improvements in efficiency and accuracy for quantum chemical calculations.
• This work enables more reliable theoretical predictions for molecular excited states, aiding in the interpretation of spectroscopic data.