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Minimum-energy crossing points (MECP) between potential energy surfaces with different spin multiplicities are important in non-radiative relaxation of excited states, analysis of phosphorescent materials and for the study of "spin-forbidden" reactions. Harvey et al. described an algorithm that you can use to find MECP using any theory level, in J.N.Harvey, M.Aschi, H.Schwarz, W.Koch (1998) <br>The Singlet and Triplet State of Phenyl Cation. A Hybrid Approach for Locating Minimum Energy Crossing Points between Non-interacting Potential Energy Surfaces. Theor. Chem. Acc. (1998) 99, 95 Prof. Harvey kindly provided me with the code, which I have aapted for use with the output from the quantum-chemistry program Firefly (Alex Granovsky, Moscow State University, http://classic.chem.msu.su/gran/gamess/index.html) mecp requires output and punch <br>files from two calculations of the same system, with the same coordinates, but different<br> spin multiplicities. These files MUST be named as: First.out and First.dat (for the first spin multiplicity, i.e. that with the HIGHER energy )<br>Second.out and Second.dat (for the second spin multiplicity)<br><br><br>mecp produces several output files: results.txt (geometries, graidents, energies, etc...)<br>results.xyz (geometries in xyz format)<br>new_geom.txt (the next geometry in the MECP optimization)<br>new_grad.txt (computed gradient - output only when BFGS update is selected)<br>hessian.txt (computed hessian - output only when BFGS update is selected)<br>vec1.txt (orbitals for the first state, taken from First.dat)<br>vec2.txt (orbitals for the second state, taken from Second.dat)<br><br><br>new_geom.txt, vec1.txt and vec2.txt must be combined in order to build input files <br>(First.txt and Second,txt) for a new round of computation. The easiest way to do this<br> is to have all other input commands in separate files (e.g. First_temp.txt and Second_temp.txt), <br>and concatenate them as needed.<br><br><br><br><br><br><br>In the first paper I published using MECP (Silva &amp; Ramos (2007), J. Phys. Chem. B,111, 12883-12887), I found that in my system the MECP <br>geometry in gas phase was quite different from the one in solution. It is therefore STRONGLY recommended that spin-forbidden reactions in solution be studied in the presence of implicit or explicit solvent models.<br><br><br> DISCLAIMER: The programs were built for my personal use. My programming skills are limited (to say the least) and I am sure that a good programmer would be able to make them much more user-friendly.