Selected configuration interaction with truncation energy error and application to the Ne atom

Area of Science:

• Quantum Chemistry
• Computational Physics
• Electronic Structure Theory

Background:

• Configuration Interaction (CI) methods are fundamental for electronic structure calculations.
• Approximating large CI expansions is crucial for computational feasibility.
• Accurate energy calculations require systematic inclusion of electron correlation effects.

Purpose of the Study:

• To reformulate Selected Configuration Interaction (SCI) into a general framework for all CI methods.
• To develop a method for approximating energy truncation errors from disconnected configurations.
• To achieve high accuracy in electronic structure calculations for atomic and molecular systems.

Main Methods:

• Utilizing linked cluster expansion to approximate CI coefficients.
• Employing Brown's energy formula for selecting disconnected configurations up to sextuply excited.
• Applying natural orbital concepts for selecting connected configurations.
• Calculating residual error through sensitivity analyses.

Main Results:

• Demonstrated SCI's applicability to Ne ground state with 1077 orbitals.
• Achieved near spectroscopic accuracy (0.5 cm(-1)) with a model space of 1.4 x 10^9 CSFs.
• Accurately determined energy contributions of high-order excitations (quintuples and sextuples).
• Developed a method to estimate orbital basis incompleteness error.

Conclusions:

• The reformulated SCI provides a robust framework for accurate electronic structure calculations.
• The method efficiently handles large configuration spaces and high-order excitations.
• SCI offers a pathway to estimate basis set incompleteness errors without full CI calculations.