Transition State Calculations in Gaussian: Berny, QST2, and QST3 Methods

Transition state (TS) calculations in Gaussian involve the use of various methods, such as the Berny optimization, QST2, and QST3 methods.

The Berny optimization method is a powerful optimization technique that can be used to locate the transition state of a chemical reaction. It is based on the use of a modified version of the steepest descent algorithm and a modified version of the conjugate gradient algorithm.

The QST2 method is a second-order quantum mechanics-based method that can be used to calculate the transition state of a chemical reaction. It is based on the use of a quadratic approximation to the potential energy surface around the transition state.

The QST3 method is a third-order quantum mechanics-based method that can be used to calculate the transition state of a chemical reaction. It is based on the use of a cubic approximation to the potential energy surface around the transition state.

In Gaussian, these methods can be specified by inputting the appropriate keywords and options in the input file before running the calculation.