Multireference exciplex binding energies: Basis set convergence and error

Abstract

In multichromophore systems, characterization of electronic structure requires characterization of exciplexes, electron‐hole pairs delocalized over multiple molecules. Computing exciplex binding energy requires an accurate description of both the noncovalent interactions between the chromophores and their excited electronic states. The critical role of basis set selection for accurate description of noncovalent interactions is well known, but for some of the most accurate excited‐state methods, basis set dependence is incompletely understood. In this work, the impact of basis set size and diffuseness on CASSCF/NEVPT2 binding energies is determined for three systems in their lowest singlet excited states: the benzene excimer, the cis‐butadiene‐benzene exciplex, and the benzene‐naphthalene exciplex. We demonstrate that excellent CBS binding energies may be obtained using the moderately‐sized jun‐cc‐pV(D + d)Z and jun‐cc‐pV(T + d)Z basis sets and a simple N−3 model. Repeating this procedure with the N = 3, 4 basis sets from the most diffuse basis set family applied to each system yields a binding energy of 56.6 ± 1.2 kJ/mol for the benzene excimer and binding energies of 11.1 ± 0.5 kJ/mol and 19.2 ± 1.7 kJ/mol for the cis‐butadiene‐benzene exciplex and the benzene‐naphthalene exciplex, respectively.

Exciplex Stabilization in Asymmetric Acene Dimers

“…Error due to the use of the DMRG approach is estimated at ≈ 1 kJ/mol based on E B convergence with M for this complex. 32 The shape and well depth of the intermolecular exciplex potential energy surface varies significantly between the TDDFT calculations performed using different exchangecorrelation functionals. The LC-BLYP functional has a binding energy 54% lower than the NEVPT2/cc-pVDZ result.…”

“…The CASSCF/CASCI/NEVPT2 techniques used here have been employed in previous work, 32 and only a brief review will be provided. This multireference procedure is used here in order to calculate complete binding curves for (BN) * , (BA) * , and (NA) * using the cc-pVDZ basis set, 36 data that is reported here for the first time.…”

“…44 The choice of starting orbital type can affect the convergence of DMRG energies with increasing M , but aromatic exciplex binding energies computed using canonical and localized starting orbitals are both generally wellconverged with M = 500 for CASSCF calculations and M = 1200 for CASCI and NEVPT2 calculations. 32 For the larger (NA) * exciplex, setting M = 1000 for the CASSCF step was necessary to obtain a converged binding energy, and localized B3LYP starting orbitals were used because RHF orbitals generated were of poor quality. For (BA) * , the binding energy calculated using both sets of conditions differed by less than 1 kJ/mol, so the full binding curve was generated using M = 500 for the CASSCF step and RHF starting orbitals.…”

“…30 The lowest singlet excited state of the benzene-naphthalene exciplex is derived from the L b state of the naphthalene monomer. An approximate CP-corrected NEVPT2 complete basis set binding energy of 19.2 kJ/mol has been reported for the complex in the S 1 state, 32 indicating a much weaker interaction than the ones observed for S benzene and naphthalene excimers.…”

Noncovalently bound excited-state dimers: a perspective on current time-dependent Density Functional Theory approaches applied to aromatic excimer models

“…168 In the case of interaction energies, BSIE is not consistent with varied inter-monomer separation and will therefore not be cancelled by similar error of infinitely separated monomers in the calculation of interaction energies. 169 If use of a larger basis set is not suitable, additive corrections have been developed for ground states to minimise the BSSE. [170][171][172] BSIE corrections 173,174 have also been developed although they are generally scare and more computationally demanding.…”