Ab Initio Geometry and Bright Excitation of Carotenoids: Quantum Monte Carlo and Many Body Green's Function Theory Calculations on Peridinin

Abstract

In this letter, we report the singlet ground state structure of the full carotenoid peridinin by means of variational Monte Carlo (VMC) calculations. The VMC relaxed geometry has an average bond length alternation of 0.1165(10) Å, larger than the values obtained by DFT (PBE, B3LYP, and CAM-B3LYP) and shorter than that calculated at the Hartree-Fock (HF) level. TDDFT and EOM-CCSD calculations on a reduced peridinin model confirm the HOMO-LUMO major contribution of the Bu(+)-like (S2) bright excited state. Many Body Green's Function Theory (MBGFT) calculations of the vertical excitation energy of the Bu(+)-like state for the VMC structure (VMC/MBGFT) provide an excitation energy of 2.62 eV, in agreement with experimental results in n-hexane (2.72 eV). The dependence of the excitation energy on the bond length alternation in the MBGFT and TDDFT calculations with different functionals is discussed.

Chart 1. Sketch Representation of Peridinin (PID) and of the Reduced Peridinin Model (PID1)

Figure 1

Comparison of the bond length alternation pattern of PID from ground state structures optimized at DFT/PBE, DFT/B3LYP, DFT/CAM-B3LYP, VMC, and Hartree–Fock levels. The VMC1 basis is defined in Table 1 of the SI and in refs 25c and .

Figure 2

B_u⁺-like and A_g⁻-like excited state energies (eV) of PID calculated by TDDFT//6-31++G** with three different functionals (SVWN, B3LYP, and CAM-B3LYP) as a function of the average BLA (Å). MBGFT results are also shown.