Introduction

OPTIM2 is the latest version of the OPTIM program for locating stationary points on potential energy surfaces and calculating reaction pathways. Optimisation algorithms now include eigenvector-following, [1,2,3,4,5,6,7] steepest-descent (via the Page-McIver method or Bulirsch-Stoer or Runga-Kutta integration), conjugate gradient and hybrids thereof. Pathways can be calculated in several ways, and it is also possible to use a `fictitious kinetic' metric, usually referred to as `mass-weighted' coordinates. There is provision to treat rigid body systems; the TIPS family of rigid molecule, effective pair potentials for HO are known to the program and employ centre-of-mass and Euler angle coordinates. An older version of the eigenvector-following optimiser is present in the ORIENT3 program, which can treat arbitrary mixtures of atoms and rigid molecules using distributed multipoles to describe the electrostatic energy.[8,9] These optimisation algorithms can also be very effective in solving fitting problems, especially if analytic derivatives can be obtained.

In OPTIM.2.3 the calculation of complete pathways was introduced for a single OPTIM run, along with automated printing of xyz pathway files, energy profiles, rate constants and pathway characteristics such as the cooperativity index. Double-ended pathway calculations were first supported in OPTIM.2.3, including the nudged elastic band approach and characterisation of paths involving multiple transition states.

The program has analytic first and second energy derivatives coded for dozens of
empirical potentials and can also read derivatives from disc so that it can be run
iteratively in tandem with *ab initio* or other packages.
The only file that you usually need to start a calculation is `odata`.
Each call to OPTIM performs one or more steps of various kinds.
The updated coordinates after every step are saved in order
in file `points`. The coordinates after the final step are also written to file
`points.final` and a new input file for OPTIM based upon this geometry is written
to `odata.new`. This file also contains the energy for the previous step and
the point group (if it was evaluated) as *COMMENT* statements.