An excited-state approach within full configuration interaction quantum Monte Carlo

Area of Science:

• Quantum Chemistry
• Computational Physics
• Materials Science

Background:

• Calculating excited states is crucial for understanding chemical reactions and material properties.
• Existing methods for excited states can be computationally expensive and complex.
• Full Configuration Interaction Quantum Monte Carlo (FCIQMC) is a powerful method for ground-state calculations.

Purpose of the Study:

• To present a novel, computationally inexpensive approach for calculating excited states.
• To enable the study of multiple low-energy states within the same symmetry using FCIQMC.
• To extend the applicability of FCIQMC to excited-state problems.

Main Methods:

• A Gram-Schmidt orthogonalization procedure applied to walker distributions in FCIQMC.
• Orthogonalization of higher energy states against lower energy states.
• Direct calculation without trial wave functions or space partitioning.

Main Results:

• The new method successfully calculates excited states with comparable computational cost to ground-state FCIQMC.
• Demonstrated application to the carbon dimer system using quadruple-zeta basis sets.
• Achieved accurate results for excited states, consistent with existing data.

Conclusions:

• The developed FCIQMC approach offers an efficient and accessible route to excited-state calculations.
• This method significantly reduces the computational barrier for studying excited states.
• It opens possibilities for investigating excited-state properties in larger and more complex systems.