Quantum computing holds a prominent position in the current central science and technology. Among the diverse topics in the field of quantum computing, quantum chemical calculations of atoms and molecules have been intensively studied as the near-future applications of quantum computers. An approach to execute full-CI calculations on quantum computers by using quantum phase estimation algorithm was reported in 2005, and a quantum-classical hybrid computation model known as a variational quantum eigensolver was proposed in 2014. Most of theoretical methods for quantum chemical calculations on quantum computers reported so far assume that the approximate wave function having sufficiently large overlap with the full-CI wave function can be easily prepared. They sometimes assume that the Hartree-Fock wave function is a good approximation of the electronic ground state. However, when molecules have open shell low-spin characters like antiferromagnetically coupled multi-nuclear transition metal complexes and molecules undergoing covalent bond dissociations, the full-CI wave function possesses multi-configurational characters to satisfy the symmetry requirement of the spin operator S2, and the overlap between Hartree-Fock and full-CI wave functions becomes small. Development of theoretical methods to prepare good initial guess wave functions for open shell low-spin electronic structures on quantum computers without performing time-consuming computations is an important issue in quantum chemical calculations on quantum computers. Here we propose a method to construct multi-configurational wave functions having large overlap with the full-CI wave function on quantum computers, without performing any post-Hartree-Fock calculations [1]. The approach uses diradical characters obtainable from the occupation number of natural orbitals at the spin-projected UHF level as the measure of open shell characters. Quantum circuits for the preparation of the multi-configurational wave functions and some applications for electronic structure calculations will be given.

Multi-configurational quantum chemistry on quantum computers

Macromolecular Chemistry: The Second Century

[Author Benefits](https://scimeetings.acs.org/?utm_source=pubs_content_marketing&utm_medium=website&utm_campaign=0320_MCF_NPI_Launch_Spring_Homepage&ref=pubs_content_marketing)

[How to use SciMeetings](https://storage.googleapis.com/dl.morressier.com/acs/SciMeetings-ACSSpring2020-HowToGuide_M3020-Final.pdf)

[Frequently Asked Questions](https://storage.googleapis.com/dl.morressier.com/acs/SciMtng_ACSNMSp2020_FAQ_M3120_Final.pdf)

ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.