Yang Yang Postdoc in theoretical chemistry at Yale University
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  • My current research project is mainly on method development in the nuclear-electronic orbital (NEO) method. This method treats both electrons and some key nuclei quantum-mechanically and describes them within molecular orbital picture. In most cases of interest, only certain hydrogen nuclei are treated quantum-mechanically because they are light but their role in chemical reactions is very important.

    Using my knowledge from my Ph.D. study, I developed an electron-proton correlation (epc) functional within the framework of NEO density functional theory (NEO-DFT). It is the first working epc functional and it can accurately reproduce the diffused proton density. We name the functional as epc17-1. I also re-paramterized the functional and make it give reasonable zero-point energies, and this form is named as epc17-2.

    Furthermore, I derived the time dependent version of NEO-DFT, which we name as NEO-TDDFT. It can provide good proton vibration energies as well as electronic excitations.

    My research project in Ph.D. is mainly on the particle-particle random phase approximation (pp-RPA), a theoretical method that is based on but also beyond the widely used density functional approximations. The pp-RPA actually has been a textbook method for treating correlation in nuclear physics for a long time, but in the recent years we introduced it to atomic and molecular systems. We also successfully combined it with density functional approximations besides the traditional Hartree-Fock approximation.

    What I work on most is to apply the pp-RPA to electronic excitation problems. These problems include challenging double excitation, charge transfer excitation, Rydberg excitation, and diradical problems. The calculation usually starts from a two-electron deficient system and then recovers a series of neutral states by adding two electrons back to the system. Although it intrinsically misses those excitations from below the highest occupied molecular orbital, it often can solve above challenging excitation problems that cannot be well described by the more widely-used adiabatic time-dependent density functional theory (TDDFT).

    The pp-RPA is in nature a theoretical counterpart of TDDFT and potentially can be quite useful. We hope it can some day become another powerful tool for solving excited states problems.

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