OPTIM3 is the latest version of the OPTIM program for locating stationary points on potential energy surfaces and calculating reaction pathways. [1] A number of changes have been introduced from the last stable release, OPTIM.2.3. In particular, it is no longer necessary to recompile the program to treat larger systems, since the dynamic memory features of fortran90 have now been used throughout. The default build target for the Makefile is OPTIM.3.2, but three other targets are now possible, namely COPTIM, UNOPTIM and AMOPTIM, which produce corresponding executables interfaced to the CHARMM,[2,3] UNRES, and AMBER95[4] potentials, respectively. Separate directories containing libraries built from modified CHARMM and UNRES sources must be present to build the corresponding COPTIM and UNOPTIM executables.
The optimisation algorithms include eigenvector-following,
[5,6,7,8,9,10,11] 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 H
O 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.[12,13] These optimisation algorithms can also be
very effective in solving fitting problems, especially if analytic derivatives
can be obtained.
The main new keywords available in OPTIM3 are NEWCONNECT and NEWNEB; the CONNECT and NEB keywords remain available, but the new algorithms will probably be more effective. The NEWNEB keyword specifies a doubly-nudged elastic band (DNEB) algorithm for double-ended searches,[14] while NEWCONNECT provides a more sophisticated way to link local minima that are likely to be connected by pathways involving a number of transition states. The GROWSTRING and EVOLVESTRING keywords provide implementations of the growing string and evolving string methods.[15,16] A new framework has also been introduced for treating rigid bodies that interact via isotropic site-site potentials using angle/axis coordinates.[17]
New in OPTIM3 is the ability to specify a file extension when OPTIM is invoked on the
command line. For example:
OPTIM.3.1 2 
& output
will result in all other output files having the
extension `.2', e.g. odata.new.2, points.final.2, etc.
In contrast to the use of the FILTH keyword, when OPTIM is invoked in this way
it will also assume that all input files carry the same extension, e.g. odata.2, finish.2.
The intention is to retire the FILTH keyword as soon as the Filthy_Phyllis program
has been rewritten appropriately. It will then be possible to use non-integer file extensions;
at the moment only integers between 1 and 999 can be handled.
The OPTIM 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 need to start simple calculations 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 last step and the point group (if it was evaluated) as COMMENT statements.