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Unitary group adapted state specific multireference perturbation theory: Formulation and pilot applications
Avijit Sen1, Sangita Sen, Pradipta Kumar Samanta
1Raman Centre for Atomic, Molecular and Optical Sciences, Indian Association for the Cultivation of Sciences, Jadavpur, Kolkata, India.
Journal of Computational Chemistry
|February 18, 2015
Summary
We introduce UGA-SSMRPT2, a new computational method for electronic structure calculations. This approach accurately models potential energy surfaces and ensures size-consistency, outperforming other methods for challenging molecular systems.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- The development of accurate and efficient computational methods is crucial for understanding molecular behavior.
- State-specific multireference coupled cluster (SSMRCC) theories offer a robust framework for treating strongly correlated electronic systems.
- Unitary Group Adapted (UGA) formalisms provide a systematic way to handle symmetries in electronic structure calculations.
Purpose of the Study:
- To present the formulation and pilot applications of the second-order perturbative analogue of UGA-SSMRCC, termed UGA-SSMRPT2.
- To investigate the similarities and differences between UGA-SSMRPT2 and the related SA-SSMRPT2.
- To assess the performance of UGA-SSMRPT2 in accurately describing potential energy surfaces (PES) and ensuring size-consistency.
Main Methods:
- Formulation of the UGA-SSMRPT2 method, a size-extensive theory.
- Application of localized orbitals to ensure size-consistency, addressing noninvariance issues.
- Pilot studies involving calculations of PES for small, difficult molecules in various low-lying states.
- Comparison of results using natural, pseudocanonical, and localized orbitals against Full CI (FCI).
- Evaluation of nonparallelity errors (NPE) and mean average deviations (MAD).
- Demonstration of size-consistency through fragmentation calculations.
Main Results:
- UGA-SSMRPT2 demonstrates size-extensivity and requires localized orbitals for size-consistency.
- Natural orbitals yield the most accurate PES, indicated by lower NPE and MAD values.
- UGA-SSMRPT2 outperforms MRMP2 and MCQDPT2 for individual states and avoided curve crossings, respectively.
- Size-consistency checks show complete insensitivity of fragment energy sums, confirming the method's robustness.
Conclusions:
- UGA-SSMRPT2 is a reliable and accurate method for electronic structure calculations, particularly for challenging molecular systems.
- The use of localized orbitals is essential for achieving manifest size-consistency in this formalism.
- The method shows significant promise for future applications in quantum chemistry and related fields.
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