Calculation of Integrated Vibrational Band Intensities for Molecules with Large Amplitude Motions

The AGAPES-TANA-MCTDH suite for the calculation of anharmonic vibrational wave functions of molecules with large amplitude motions is extended to include infrared intensities. In our approach, fully converged dipole moment surfaces are generated by AGADMS (Adaptive Generation of Adiabatic Dipole Moment Surfaces)─a modification of AGAPES─in an Eckart-Sayvetz type frame to generate the transition dipole moments using the anharmonic vibrational wave functions from J = 0 MCTDH improved relaxation calculations. The J = 0 integrated absolute band intensities A_0→f for two molecules exhibiting large amplitude motions─HCOOH and H₂NOH─are presented at the SOGGA11-X/aug-cc-pVDZ level for the dipole moment surface and at the CCSD(T) level anharmonic for the vibrational eigenfunctions from previous works. An indeterminacy in the definition of the flexible Eckart-Sayvetz reference is discussed and exploited to remove a large torsion rotation coupling and the huge errors that come along with it for $A_{0\to {\nu }_9}$ of NH₂OH.