Ab Initio Composite Methods

Angela K. Wilson, Wanyi Jiang, Gbenga A. Oyedepo +1 more

Dharam Vir Ahluwalia

Yueqing Chang, Sonali Joshi, Lucas K. Wagner

We present a generalized framework, renormalized density matrix downfolding (RDMD), to derive systematically improvable, highly accurate, and nonperturbative effective models from ab initio calculations. This framework moves beyond the common role of ab initio calculations as calculating the parameters of a proposed Hamiltonian. Instead, RDMD provides the capability to decide whether a given effective Hilbert space can be identified from the ab initio data and assess the relative quality of ansatz Hamiltonians. Any method of ab initio solution can be used as a data source, and as the ab initio solutions improve, the resultant model also improves. We demonstrate the framework in an application to the downfolding of a hydrogen chain to a spin model, in which we find the interatomic separations for which a nonperturbative mapping can be made even in the strong coupling regime where standard methods fail, and compute a renormalized spin model Hamiltonian that quantitatively reproduces the ab initio dynamics.

Alexander Quandt

Ghania Boucekkine, Abdou Boucekkine

Xiaomeng Liu, Yonggang Yang, Jianming Zhao +2 more

An efficient method to study highly excited states at the ab initio level has been introduced and applied to ultralong Rydberg CsNe molecules. Vibrational properties of Rydberg CsNe molecules are investigated on corresponding potential energy curves obtained by perturbation theory. The Rydberg CsNe molecules are associated with a Rydberg Cs(ns∕np) atom (n = 20-60) and a ground state Ne((1)S0) atom. The starting point for the perturbation treatment of corresponding Rydberg molecular potential energy curves is to generate accurate atomic Rydberg states from realistic ab initio effective core potential. According to the authors' knowledge this is a good reference for ultralong range molecules (order of 1000 Bohr radii) to be studied at the ab initio level.