Searches for molecular species on the potential energy surfaces of HCN, H 3 CO 2 and H 4 CF by the dynamically defined reaction path (DDRP) method 1 Gyula Do ¨mo ¨to ¨r a , La ´szlo ´ L. Stacho ´ b , Miklo ´s I. Ba ´n a, * a Institute of Physical Chemistry, Attila Jo ´zsef University, PO Box 105, H-6701 Szeged, Hungary b Bolyai Institute for Mathematics, Attila Jo ´zsef University, Aradi Ve ´rtanu ´k tere 1, H-6720 Szeged, Hungary Received 26 May 1997; accepted 5 August 1997 Abstract By using the global path-following dynamically defined reaction path (DDRP) method coupled with the semiempirical quantum chemical MNDO procedure, molecular species and conformers found on the potential energy surfaces of the chemical systems HCN, H 3 CO 2 and H 4 CF have been identified and investigated extensively and comparisons with literary results of other theoretical methods and experiments have been made.  1998 Elsevier Science B.V. All rights reserved. Keywords: Potential energy surfaces; Path-following; Saddle point; DDRP method 1. Introduction Fukui’s reaction path (RP), the intrinsic reaction coordinate (IRC) [1], was originally defined as the union of the steepest descent paths from the transition state (TS) to the reactant and product minima, using mass-weighted coordinates, or as an imaginary mini- mum energy reaction path which passes through the TS and moves infinitely slow. Even in systems of small molecules several chemical reactions and/or conforma- tional changes can take place, therefore, also bifurca- tions and multiple branchings of RPs may occur. This leads on the potential energy hypersurface (PES) to a complicated network of RPs with minima and maxima/ saddle points (SPs), the accurate exploration of which encounters great difficulties both for computational reasons (e.g. mainly for the convergence problems of the method used to calculate the gradients/energies) and – in close connection with the former – for financial ones (e.g. for the extraordinarily high computational expenditures). This is why the local (direct) path- following (PF) methods [2] have been advantages and preferences so far, over the global (indirect) methods [3]. However, the latter methods may still have ade- quate grounds for use because, in theory, when explor- ing the network of reaction routes and the connecting points and couplings of the branches of various ramify- ing RPs and reaction steps, one cannot expect success at all from other than global methods. Another aspect is the stability of the actual PF method. Even in this respect the global methods are generally superior to local ones. The global dynamically defined reaction path (DDRP) method [4] fulfils these requirements, so it has been applied to the scrutinies of the systems Journal of Molecular Structure (Theochem) 455 (1998) 219–228 0166-1280/98/$ - see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0166-1280(98)00117-1  In honor of Professor A ´ rpa ´d Kucsman on the occasion of his 70th birthday. 1 This work was presented in part at the 4th Congress of WATOC (WATOC’96); July 7–12, 1996 Jerusalem, Israel, Program and Abstracts, pp. 266, 267. * Corresponding author.