RT3: A Windows program for the Renner–Teller analysis of Π2 states of triatomic molecules

In this work we present Windows and Fortran programs which can be used for the Renner–Teller analysis of the vibronic levels of Π2 states of linear, triatomic molecules. The programs can do least squares fitting of term values relative to the lowest energy level within a single Π2 state, of combination differences between vibronic levels within a single Π2 state or of transitions between the levels of two different Π2 states. The algorithm allows for the inclusion of Renner–Teller, spin–orbit, vibrational anharmonicity, Fermi resonance and Sears resonance terms in the Hamiltonian matrices. The Windows program RT3, written in Visual Basic 6.0 with a Fortran DLL engine for the numerically intensive computations, facilitates the construction and editing of input files and comparisons of input and output. For very large calculations, the Fortran code may be run as a stand-alone program on mainframe or other computers, but the Windows version offers significant advantages for common usage.Program summaryTitle of program: RT3Catalogue identifier: AEAL_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEAL_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University of Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 37 477No. of bytes in distributed program, including test data, etc.: 3 665 414Distribution format: tar.gzProgramming language: Visual Basic 6.0, Fortran 90Operating systems: Windows 98, Windows XP, Windows Vista, UNIXClassification: 16.2Nature of problem:   The vibronic levels of triatomic molecules in Π2 states are complicated by spin–orbit and vibrational-orbital angular momentum coupling (Renner–Teller) effects that make them difficult to assign and analyze. The RT3 program, available in Windows and Fortran versions, can be used to calculate the vibronic energy levels of such systems and fit experimental data to a Hamiltonian that includes Renner–Teller, spin–orbit, vibrational anharmonicity, Fermi resonance and Sears resonance effects.Solution method:   The RT3 program solves for the Renner–Teller levels of a Π2 state by diagonalizing large truncated Hamiltonian matrices whose elements are generated in a harmonic oscillator basis set. Given an initial input set of constants, the resulting energy levels can be used to least squares fit or predict vibronic term values (relative to the lowest energy level), combination differences between vibronic levels within a electronic state, and transitions between the vibronic levels of two different Π2 states.Running time: 6.0 S to run the Transitions.ir3 example file on an Intel Pentium 4 2 GHz CPU with the Windows XP operating system.