MSTor 2023: A new version of the computer code for multistructural torsional anharmonicity, now with automatic torsional identification using redundant internal coordinates

Abstract

The MSTor program is a computer program for calculating partition functions and thermodynamic functions of complex gas-phase molecules with multiple torsions; the multi-structural approximation with torsional anharmonicity (MS-T) can be used based on either a coupled torsional potential or an uncoupled torsional potential. The program can also carry out calculations in the multiple-structure local harmonic (MS-LH) approximation or multi-structural local quasiharmonic (MS-LQ) approximation or by the dual-level MS-T method. Furthermore, the program package includes eight utility codes that can be used as stand-alone programs to help the user to generate the input files for the code or to generate comparison results. The 2023 version of MSTor includes a new capability, namely automatic identification of torsional modes with redundant internal coordinates. New version program summaryProgram title: MSTor 2023CPC Library link to program files:https://doi.org/10.17632/t4y5wds856.1Licensing provisions: Apache 2.0Programming language: Fortran 90, C, and PerlJournal references of previous versions: Comput. Phys. Comm. 183 (2012) 1803; Comput. Phys. Comm. 184 (2013) 2032Does the new version supersede the previous version?: Yes.Nature of problem: Calculation of the partition functions and thermodynamic functions (standard-state energy, enthalpy, entropy, and free energy as functions of temperature) of molecules or transition states involving multiple torsional motions.Solution method: The program uses the multi-structural approximation with torsional anharmonicity, a coupled torsional potential, and delocalized torsions MS-T(CD) [1]. The program also provides results for the multi-structural local harmonic approximation or local quasiharmonic approximation, and it can also be used for the original multi-structural approximation with torsional anharmonicity and a coupled torsional potential MS-T(C) [2] or an uncoupled torsional potential MS-T(U) [3]. The program can also carry out dual-level [4] MS-T(CD), MS-T(C), and MS-T(U) calculations.Reasons for new version: The new version increases functionality and is more user friendly. The main enhancement is the capability for automatic identification of torsional modes with redundant internal coordinates.Summary of revisions: A newly developed torsional identification approach for complex molecules has been implemented in the MSTor program. The method, called multi-structural torsion method with a coupled torsional potential and delocalized torsions MS-T(CD), is based on Baker et al.'s delocalized internal-coordinates method and on Zheng and Truhlar's torsional projection method. By using a redundant-internal-coordinate auto-generation procedure, the MS-T(CD) method circumvents the need in the original MS-T method for the user to define nonredundant internal coordinates, and it can straightforwardly identify and separate the coupled torsions from the Cartesian Hessian. This greatly simplifies the user input, and it helps users to obtain robust and consistent results. MSTor now sets the redundant internal coordinate scheme as the default method.A new utility code, DLMSTor.exe, is provided to allow dual-level calculations. The $framechain and $framedef sections were reintroduced to the code as optional sections. If these sections are used, the D matrix is calculated using the scheme of Kilpatrick and Pitzer and one should always get correct values for the moments of inertia.Various utility codes were modified to help ensure that structures that are equivalent under rotation to other structures (and thus indistinguishable when considering overall rotation and torsions) are still properly included in the Voronoi tessellation step (which does not fully exploit overall rotational symmetry).The mcvorm.exe utility code was modified to correctly calculate uncertainties and to take advantage of rotational symmetries (at least for linear-chain molecules) and mirror-image symmetries, if present, in the calculation of the uncertainty estimates.We added a new keyword, MTUMME, that provides an option to write a density-of-states file for version 2023 (in preparation) of the TUMME [5] master equation program.Some minor bugs are fixed so the code now compiles properly with the GNU gfortran compiler (version 12.2.1), the NVidia HPC Fortran compiler pgfortran (version 21.3), and the Intel compiler ifort. The output format is cleaned, and unused variables and arrays are removed. The new version has an increased number of test runs, and the manual is improved and updated.The program authors are J. Zheng, W. Chen, S. L. Mielke, J. L. Bao, R. Meana-Pañeda, K. L. Clarkson, X. Xu, and D. G. Truhlar; the new version announcement authors are W. Chen, J. Zheng, J. L. Bao, D. G. Truhlar, and X. Xu; the primary corresponding author is X. Xu.