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Manuscript Title: CMIstark: Python package for the Stark-effect calculation and symmetry classication of linear, symmetric and asymmetric top wavefunctions in dc electric fields
Authors: Yuan-Pin Chang, Frank Filsinger, Boris G. Sartakov, Jochen Küpper
Program title: CMIstark
Catalogue identifier: AEQS_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 185(2014)339
Programming language: Python (version 2.6.x, 2.7.x).
Computer: Any Macintosh, PC, or Linux/UNIX workstations with a modern Python distribution.
Operating system: Tested on Mac OS X and a variety of Linux distributions.
RAM: 2 GB for typical calculations
Keywords: Molecular rotation, Linear top molecule, Symmetric top molecule, Asymmetric top molecule, Electric field, Stark effect.
Classification: 16.1.

External routines: Python packages numpy and scipy; utilizes (optimized) LAPACK and BLAS through scipy. All packages available under open-source licenses.

Nature of problem:
Calculation of the Stark effect of asymetric top molecules in arbitrarily strong dc electric fields in a correct symmetry classification and using correct labeling of the adiabatic Stark curves.

Solution method:
We set up the full M matrices of the quantum-mechanical Hamiltonian in the basis set of symmetric top wavefunctions and, subsequently, Wang transform the Hamiltonian matrix. We separate, as far as possible, the sub-matrices according to the remaining symmetry, and then diagonalize the individual blocks. This application of the symmetry consideration to the Hamiltonian allows an adiabatic correlation of the asymmetric top eigenstates in the dc electric field to the field-free eigenstates. This directly yields correct adiabatic state labels and, correspondingly, adiabatic Stark energy curves.

The maximum value of J is limited by the available main memory. A modern desktop computer with 16 GB of main memory allows for calculations including all Js up to a values larger than 100 even for the most complex cases of asymmetric tops.

Running time:
Typically 1 s-1 week on a single CPU or equivalent on multi-CPU systems (depending greatly on system size and RAM); parallelization through BLAS/LAPACK. For instance, calculating all energies up to J = 25 of indole (vide infra) for one field strength takes 1 CPU-s on a current iMac.