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Applications & Benchmarks Working Group Monthly Seminars
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GronOR: Non-Orthogonal Configuration Interaction
Tjerk P. Straatsma and Coen de Graaf
GronOR is a non-orthogonal configuration interaction (NOCI) application, developed by the University of Groningen, Oak Ridge National Laboratory and University Rovira i Virgili. The target scientific application is the description of the electronic structure of molecular assemblies in terms of basis functions that can be interpreted as a particular combination of molecular electronic states. The electronic states obtained in this basis can be interpreted directly in terms of molecular states, and with appropriate unitary transformations, the canonical molecular orbitals can be transformed into a description resembling the valence bond picture for the description of the electronic structure of molecules in terms of Lewis structures. The method would can also allow an accurate description of processes that occur locally, like excitation of one molecule in the nanostructure. The basic methodology consists of generation of spin adapted, antisymmetrized, combinations (SAACs) of molecular wavefunctions, followed by a non-orthogonal configuration interaction (NOCI) calculation using these SAACs as many-electron basis functions (MEBFs). The NOCI wavefunction of a cluster/ensemble of molecules is written as a linear combination of MEBFs, each formed as a SAAC of the wavefunctions for a particular state for each molecule in the ensemble. Using fully relaxed molecular electronic states and correlated molecular wavefunctions has the advantage that orbital relaxation and local correlation effects can be properly included in the description of the locally excited states, while avoiding lengthy CI expansions. The implementation of the NOCI method in GronOR is interfaced with OpenMolcas to obtain the CASSCF CI vector, the state specific CASSCF orbitals, and the required two-electron integrals. The evaluation of the Hamiltonian matrix elements involves many contributions in the form of determinant pairs that can be calculated independently. For the massively parallel implementation of the algorithm we adopted a task-based approach with a master/worker model to achieve load balancing and fault-resilient execution. (http://www.gronor.org)
FMM & MSM: Algorithms for Long-Range Calculations in Classical Molecular Dynamics
Rio Yokota
In classical molecular dynamics simulations, the electrostatic (Coulomb) potential induces a global interaction between atoms. When calculated directly, this requires a computational cost of O(N^2) for N atoms. A common fast algorithm for calculating electrostatic forces is the particle-mesh Ewald (PME) method, which derives its speed from the efficiency of FFT for problems with high uniformity. The recent trend in hardware architectures with increasing parallelism poses a challenge for these FFT-based algorithms. Therefore, alternative algorithms such as the fast multipole method (FMM) and multilevel summation (MSM) are being considered. If we are to transition to such alternatives, a common interface between these alternatives must be developed. The developers of NAMD and GROMACS are interested in this approach.
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