Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Marco Verdicchio Applicant's institution: ANL Applicant's division: CSE Project Name: LowF_VRC-TST Project title: Low frequencies modes coupling in Variable Reaction Coordinate TST Associated funding: DOE-BES Other Systems: pople.tcg.anl.gov, linus.tcg.anl.gov Science: Radical-radical association reactions and unimolecular dissociations to free radicals are known to be of great importance in the chemistry of hydrocarbons combustion and planetary atmospheres. This kind of reactions, however, often show very low or absent local potential energy barrier, which makes traditional Transition State Theory (TST) calculations unfeasible. The variable reaction coordinate (VRC) TST approach has been proven to be of considerable utility in estimating the kinetic of barrierless reactions. The transition state dividing surface, defined by a fixed distance between a set of pivot points on each fragment, provide a valuable approximate division of the phase space into reactants and products. The position of each pivot in the molecular frame of the corresponding fragment determines the shape of the dividing surface. The optimization of both pivot point locations and distances between them provides a great flexibility in the definition of the dividing surface and allow the investigation of multiple binding sites system (i.e. addition reactions of resonantly stabilized, multiple side attack radical additions, etc.). A common problem for reactions involving radicals is the presence of a low frequency umbrella mode that is poorly treated in the harmonic oscillator approximation. Moreover this mode is often coupled to torsional modes and it is not clear how this coupling affect the partition functions. Our goal, in this project, is to develop and apply a new computational method to efficiently handle the large anharmonicities arising in molecule with strong coupled low frequencies modes. We propose to explore these issues for a number of prototypical radical-radical association reactions relevant in combustion and atmospheric chemistry. Project description: - Computational methods: We intend to use VRC-TST theory to study the effect of coupled low frequencies motions on EJ-resolved, microcanonical and canonical kinetic constants for several radical-radical association reactions. In this first stage of the project*, the following systems will be taken into consideration: - C2H5 + H - CH3 + CH3 - CH3 + CH2OH - C2H5 + O2 (*Extension and optimization of the method for larger chemical systems is also a crucial point.) The central focus of VRC-TST is the evaluation of the reactive flux through an arbitrary dividing surface for several temperatures. In particular a multifaceted dividing surface approach, which allow for the incorporation of multiple dividing surfaces for each binding site, will be adopted. This scheme allows high flexibility on the dividing surface shape and the possibility to take into account additional reaction pathways (i.e. H atom abstraction). Unfortunately the computational cost of each kinetic calculation increases with the number of considered dividing surfaces. A variational minimization of the reactive flux is then performed with respect to both the location of the pivot points and the distances between them. In order to evaluate the effect of the coupling of the two low frequencies modes on the kinetic constant we will compute the reactive flux for different values of torsional angle (Phi) and umbrella bending (Chi). This step requires the generation of a large set of independent VRC-TST kinetic calculations one for each value of the couple [Phi, Chi]. The final kinetic constant is then obtained by integrated (with proper weight functions for each point) over Phi and Chi. To provides a suitable description of the torsional and umbrella motion potentials, and to include Complete Basis Set limit corrections to the potential energy, preliminary quantum chemistry ab initio calculations are also necessary (about 10000 cpu/hours). - Technical details: Kinetic calculations will be performed using the VRC-TST method as implemented in the computer code VaReCoF (Y. Georgievskii, S.J. Klippenstein, VaReCoF, Sandia National Laboratories and Argonne National Laboratory, 2006). The code makes use of the MOLPRO or GAUSSIAN quantum chemistry software for on-the-fly evaluation of the potential energy. A Python interface to the code has been implemented for the management of the input files (reference energy evaluation, pivot points positions, etc.), for the generation of the input different structures ([Phi,Chi] grid of points) and for results post processing. The code does not require large amount of memory (less than 1GB) and works very efficiently on parallel architectures. The code makes use of MPI and shows a speed-up efficiency of about 60% for up to 96 CPU’s (test performed on a 8 cpu/cores AMD Operton 2354 machine). Improvement is expected with better CPUs and fastest internode connections. - Expected number of project members: 3 Project URL: Requested allocation: 85000 Q1: 0 Q2: 0 Q3: 35000 Q4: 50000 Justification: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System