Higher order PT terms in molecular parameters refer to the contributions from higher order derivatives of the potential energy surface (PES) in the calculation of molecular properties. These terms are usually neglected in traditional quantum chemical calculations due to their relatively small contributions, but can become important for highly accurate calculations or for systems with strong anharmonicities.
The inclusion of higher order PT terms in molecular parameters can improve the accuracy of calculated properties, especially for systems with strong anharmonicities. These terms can also provide insights into the anharmonicity of a molecule and its potential energy surface.
The most commonly used methods for calculating higher order PT terms are perturbation theory (PT) and coupled-cluster theory (CC). These methods involve the calculation of higher order derivatives of the PES, which can be computationally demanding, but can be approximated using numerical differentiation techniques.
No, the importance of higher order PT terms in molecular parameters depends on the anharmonicity of the molecule and the level of accuracy required for the calculation. For most small molecules, these terms can be neglected without significant loss of accuracy.
One way to determine the importance of higher order PT terms is to perform calculations with and without their inclusion and compare the results. If the difference in the calculated properties is significant, then the higher order PT terms should be included in the calculation.