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Geometry optimization of radicaloid systems using improved virtual orbital-complete active space configuration
Sudip Chattopadhyay1, Rajat K Chaudhuri, Karl F Freed
1Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, India. sudip_chattopadhyay@rediffmail.com
The Journal of Physical Chemistry. A
|July 1, 2010
Summary
The improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) method accurately computes radicaloid geometries. This computationally tractable approach offers an alternative to CAS self-consistent field (CASSCF) methods, overcoming convergence issues.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- Standard many-body approaches struggle with the strong multireference character of radicaloid species.
- The CAS self-consistent field (CASSCF) method, while effective, can suffer from convergence problems and high computational cost.
- The improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) method offers a computationally tractable alternative to CASSCF.
Purpose of the Study:
- To assess the performance of the analytical energy gradient IVO-CASCI approach for computing the equilibrium geometries of organic radicaloid species.
- To evaluate the accuracy of IVO-CASCI against benchmark ab initio calculations for challenging multireference systems.
- To demonstrate the practical applicability of IVO-CASCI for studying radicaloid geometries.
Main Methods:
- Application of the analytical energy gradient IVO-CASCI method.
- Computation of equilibrium geometries for diradicals (trimethylenemethane, 2,6-pyridyne) and a triradical (1,2,3-tridehydrobenzene).
- Utilized various basis sets and active space choices, treating both closed- and open-shell molecules.
Main Results:
- IVO-CASCI accurately predicted monocyclic structures for 2,6-pyridyne and its cation, contrasting with incorrect bicyclic predictions from SCF and CCSD methods.
- The method demonstrated consistent agreement with expensive benchmark ab initio calculations for radicaloid geometries.
- IVO-CASCI structures and relative stabilities for 1,2,3-tridehydrobenzene's ground and excited states aligned with experimental and theoretical data.
Conclusions:
- The IVO-CASCI analytical gradient method accurately describes radicaloid geometries, even with small reference spaces.
- This method provides a practical and computationally efficient avenue for studying diverse radicaloid species.
- IVO-CASCI overcomes CASSCF convergence issues while retaining essential benefits, making it a valuable tool in computational chemistry.
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