A reduced-restricted-quasi-Newton–Raphson method for locating and optimizing energy crossing points between two potential energy surfaces

We present a method for the location and optimization of an intersection energy point between two potential energy surfaces. The procedure directly optimizes the excited state energy using a quasi-Newton–Raphson method coupled with a restricted step algorithm. A linear transformation is also used for the solution of the quasi-Newton–Raphson equations. The efficiency of the algorithm is analyzed and demonstrated in some examples. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 :992–1003, 1997