10.2 Improved Algorithms for Transition-Structure Optimization

10.2.2 Hessian-Free Transition-State Search

Once a guess structure to the transition state is obtained, standard eigenvector-following methods such as Baker’s partitioned rational-function optimization (P-RFO) algorithm46 can be employed to refine the guess to the exact transition state. The reliability of P-RFO depends on the quality of the Hessian input, which enables the method to distinguish between the reaction coordinate (characterized by a negative eigenvalue) and the remaining degrees of freedom. In routine calculations therefore, an exact Hessian is determined via frequency calculation prior to the P-RFO search. Since the cost of evaluating an exact Hessian typically scales one power of system size higher than the energy or the gradient, this step becomes impractical for systems containing large number of atoms.

The exact Hessian calculation can be avoided by constructing an approximate Hessian based on the output of FSM.832 The tangent direction at the transition state guess on the FSM string is a good approximation to the Hessian eigenvector corresponding to the reaction coordinate. The tangent is therefore used to calculate the correct eigenvalue and corresponding eigenvector by variationally minimizing the Rayleigh-Ritz ratio.506 The reaction coordinate information is then incorporated into a guess matrix which, in turn, is obtained by transforming a diagonal matrix in delocalized internal coordinates45, 267 to Cartesian coordinates. The resulting approximate Hessian, by design, has a single negative eigenvalue corresponding to the reaction coordinate. This matrix is then used in place of the exact Hessian as input to the P-RFO method.

Example 10.6  An example one-shot, Hessian-free approach that combines the FSM and P-RFO methods in order to determine the exact transition state from reactant and product structures.

$molecule
   0  1
   Si   1.028032  -0.131573  -0.779689
   H    0.923921  -1.301934   0.201724
   H    1.294874   0.900609   0.318888
   H   -1.713989   0.300876  -0.226231
   H   -1.532839   0.232021   0.485307
****
   Si   0.000228  -0.000484  -0.000023
   H    0.644754  -1.336958  -0.064865
   H    1.047648   1.052717   0.062991
   H   -0.837028   0.205648  -1.211126
   H   -0.855603   0.079077   1.213023
$end

$rem
   JOBTYPE       fsm
   FSM_NGRAD     3
   FSM_NNODE     18
   FSM_MODE      2
   FSM_OPT_MODE  2
   METHOD        b3lyp
   BASIS         6-31g
   SYMMETRY      false
   SYM_IGNORE    true
$end

@@@

$rem
   JOBTYPE               ts
   SCF_GUESS             read
   GEOM_OPT_HESSIAN      read
   MAX_SCF_CYCLES        250
   GEOM_OPT_DMAX         50
   GEOM_OPT_MAX_CYCLES   100
   METHOD                b3lyp
   BASIS                 6-31g
   SYMMETRY              false
   SYM_IGNORE            true
$end

$molecule
   read
$end