Multicomponent MP4 and the inclusion of triple excitations in multicomponent many-body methods.

This study implements the full multicomponent third-order (MP3) and fourth-order (MP4) many-body perturbation theory methods for the first time. Previous multicomponent studies have only implemented a subset of the full contributions, and the present implementation is the first multicomponent many-body method to include any connected triples contribution to the electron-proton correlation energy. The multicomponent MP3 method is shown to be comparable in accuracy to the multicomponent coupled-cluster doubles method for the calculation of proton affinities, while the multicomponent MP4 method is of similar accuracy as the multicomponent coupled-cluster singles and doubles method. From the results in this study, it is hypothesized that the relative accuracy of multicomponent methods is more similar to their single-component counterparts than previously assumed. It is demonstrated that for multicomponent MP4, the fourth-order triple-excitation contributions can be split into electron-electron and electron-proton contributions and the electron-electron contributions ignored with very little loss of accuracy of protonic properties.

Systematic Investigation of the Molecular and Electronic Structure of Thorium and Uranium Phosphorus and Arsenic Complexes.

In continuing to examine the interaction of actinide-ligand bonds with soft donor ligands, a comparative investigation with phosphorus and arsenic was conducted. A reaction of (C5Me5)2AnMe2, An = Th, U, with 2 equiv of H2AsMes, Mes = 2,4,6-Me3C6H2, forms the primary bis(arsenido) complexes, (C5Me5)2An[As(H)Mes]2. Both exhibit thermal instability at room temperature, leading to the elimination of H2, and the formation of the diarsenido species, (C5Me5)2An(η2-As2Mes2). The analogous diphosphido complexes, (C5Me5)2An(η2-P2Mes2), could not be synthesized via the same route, even upon heating the bis(phosphido) species to 100 °C in toluene. However, they were accessible via the reaction of dimesityldiphosphane, MesP(H)P(H)Mes, with (C5Me5)2AnMe2 at 70 °C in toluene. When (C5Me5)2AnMe2 is reacted with 1 equiv of H2AsMes, the bridging µ2-arsinidiide complexes [(C5Me5)2An]2(µ2-AsMes)2 are formed. Upon reaction of (C5Me5)2UMe2 with 1 equiv of H2PMes, the phosphinidiide [(C5Me5)2U(µ2-PMes)]2 is isolated. However, the analogous thorium reaction leads to a phosphido and C-H bond activation of the methyl on the mesityl group, forming {(C5Me5)2Th[P(H)(2,4-Me2C6H2-6-CH2)]}2. The reactivity of [(C5Me5)2An(µ2-EMes)]2 was investigated with OPPh3 in an effort to produce terminal phosphinidene or arsinidene complexes. For E = As, An = U, a U(III) cation-anion pair [(C5Me5)2U(η2-As2Mes2)][(C5Me5)2U(OPPh3)2] is isolated. The reaction of [(C5Me5)2Th(µ2-AsMes)]2 with OPPh3 does not result in a terminal arsinidene but, instead, eliminates PPh3 to yield a bridging arsinidiide/oxo complex, [(C5Me5)2Th]2(µ2-AsMes)(µ2-O). Finally, the combination of [(C5Me5)2U(µ2-PMes)]2 and OPPh3 yields a terminal phosphinidene, (C5Me5)2U(═PMes)(OPPh3), featuring a short U-P bond distance of 2.502(2) Å. Electrochemical measurements on the uranium pnictinidiide complexes demonstrate only a 0.04 V difference with phosphorus as a slightly better donor. Magnetic measurements on the uranium complexes show more excited-state mixing and therefore higher magnetic moments with the arsenic-containing compounds but no deviation from uncoupled U(IV) behavior. Finally, a quantum theory of atoms in molecules analysis shows highly polarized actinide-pnictogen bonds with similar bonding characteristics, supporting the electrochemical and magnetic measurements of similar bonding between actinide-phosphorus and actinide-arsenic bonds.

Multicomponent CASSCF Revisited: Large Active Spaces Are Needed for Qualitatively Accurate Protonic Densities.

Multicomponent methods seek to treat select nuclei, typically protons, fully quantum mechanically and equivalent to the electrons of a chemical system. In such methods, it is well-known that due to the neglect of electron-proton correlation, a Hartree-Fock (HF) description of the electron-proton interaction catastrophically fails leading to qualitatively incorrect protonic properties. In single-component quantum chemistry, the qualitative failure of HF is normally indicative of the need for multireference methods such as complete active space self-consistent field (CASSCF). While a multicomponent CASSCF method was implemented nearly 20 years ago, it is only able to perform calculations with very small active spaces (∼105 multicomponent configurations). Therefore, in order to extend the realm of applicability of the multicomponent CASSCF method, this study derives and implements a new two-step multicomponent CASSCF method that uses multicomponent heat-bath configuration interaction for the configuration interaction step, enabling calculations with very large active spaces (up to 16 electrons in 48 orbitals). We find that large electronic active spaces are needed to obtain qualitatively accurate protonic densities for the HCN and FHF- molecules. Additionally, the multicomponent CASSCF method implemented here should have further applications for double-well protonic potentials and systems that are inherently electronically multireference.

Separation of electron-electron and electron-proton correlation in multicomponent orbital-optimized perturbation theory.

The multicomponent orbital-optimized second-order Møller-Plesset perturbation theory (OOMP2) method is the first multicomponent MP2 method that is able to calculate qualitatively accurate protonic densities, protonic affinities, and geometrical changes due to nuclear quantum effects in multicomponent systems. In this study, two approximations of the multicomponent OOMP2 method are introduced in an effort to demonstrate that, in orbital-optimized multicomponent methods, performing the orbital-optimization process with only electron-proton correlation is sufficient to obtain accurate protonic properties. Additionally, these approximations should reduce the computational expense of the multicomponent OOMP2 method. In the first approximation, the first-order wave function is written as a linear combination of one-electron one-proton excitations rather than as a linear combination of one-electron one-proton and two-electron excitations as in the original multicomponent OOMP2 method. Electron-electron correlation is included perturbatively after the orbital-optimization procedure has converged. In the second approach, the first approximation is further modified to neglect all terms in the orbital-rotation gradients that depend on the two-electron molecular-orbital integrals, which, assuming a fixed-sized protonic basis set, reduces the computational scaling for the orbital-optimization iterations to Ne 3, where Ne is a measure of the electronic system size, compared to the Ne 5 scaling of the original multicomponent OOMP2 method. The second approximation requires one Ne 5 step after orbital convergence to compute the electron-electron correlation energy. The accuracy of the calculated protonic densities, protonic affinities, and optimized geometries of these approximations is similar or improved relative to the original multicomponent OOMP2 method.