Mapping the Excited State Potential Energy Surface of a Retinal Chromophore Model with Multireference and...

Area of Science:

• Computational Chemistry
• Photochemistry
• Molecular Spectroscopy

Background:

• The photoisomerization of the retinal chromophore in visual pigments is crucial for vision.
• This process involves complex coupled electronic and nuclear motions on a multidimensional potential energy surface.
• Accurate computational methods are needed to model these changes.

Purpose of the Study:

• To compare the accuracy of different ab initio methods in describing the excited-state isomerization paths of a minimal retinal chromophore model, penta-2,4-dieniminium (PSB3) cation.
• To validate computational approaches against benchmark calculations.

Main Methods:

• Utilized CASSCF and CASPT2 levels of theory to investigate excited-state isomerization paths.
• Employed MRCISD+Q calculations as a benchmark for energy profiles.
• Compared MRPT2 (CASPT2, QD-NEVPT2, XMCQDPT2) and EOM-SF-CC (EOM-SF-CCSD, EOM-SF-CCSD(dT)) methods against the benchmark.

Main Results:

• CASSCF and CASPT2 methods yield topologically and energetically distinct isomerization paths.
• CASPT2 reveals a 'locally excited' region involving single bond torsion, absent in CASSCF.
• MRPT2 and EOM-SF-CCSD(dT) methods quantitatively describe the photoisomerization process.

Conclusions:

• Computational methods like MRPT2 and EOM-SF-CCSD(dT) are effective for studying visual pigment photochemistry.
• Understanding these complex reaction coordinates is key to elucidating visual pigment function.