RT1: Electronic Structure

To understand and predict quantum effects in complex systems and their interactions with external fields, we will develop accurate, scalable electronic-structure models, with emphasis on coupled-cluster theory and density-functional theory. 

Our research comprises three classes of electronic-structure methods

  • many-body methods: accurate but expensive, for dozens of atoms
  • density-functional theory (DFT): affordable but less reliable, for hundreds of atoms
  • semi-empirical methods: inexpensive but highly parameterized, for thousands of atoms 

with emphasis on aspects relevant for complex systems and matter–field interactions.

Figure of electronic structure.
Left panel: Improved reliability and affordability of the three classes of electronic-structure methods. Right panel: Bench-marking of CDFT against CCSD(T) (broad grey curve) for the dissociation of magnetically bonded Ne2 in magnetic field of 235 kT perpendicular to the molecular axis. The various exchange–correlation functionals exhibit widely different behaviours, the current- dependent cTPSS and cTPSSh functionals performing remarkably well [1]

Electronic structure sits at the core of the centers activities, and novel method developments will directly impact the research of the other five RTs.

RT1 involve developments for several software packages including DALTON [2], MOLCAS [3], DIRAC [4], OPENRSP [5], LONDON [6], MRCHEM [7] and RESPECT [8].

Many-body methods

The toolbox of many-body methods offers a systematic approach to the exact solution of many-body problems, with coupled-cluster (CC) theory the most successful of these to date. Further development of many-body methods is central for a deep understanding of electronic structure, and is vital for benchmarking DFT and semi-empirical methods – in particular when entering new and unchartered domains of complex systems and enviroments.

As part of RT1 we plan to

  • extend the DFT-based response framework of OPENRSP to CC theory in order to go beyond the state of the art for property studies (RT3)
  • extend the open-shell functionality of LONDON to equation-of-motion CC (EOM-CC) and to multiconfiguration self-consistent-field (MCSCF) theories (RT4)
  • develop a multi-reference CC formulation using the bivariational principle (BIVAQUM, ERC-StG, Simen Kvaal)
  • exporting CC theory to periodic systemsto enable high-accuracy studies of condensed-phase systems (Thomas Bondo Pedersen, RCN-RP)

Density-functional theory

The indisputable work-horse of electronic-structure theory is Kohn–Sham DFT, and as part of RT1 it will be developed to allow calculations of complex systems in complex enviroments, including

  • high-order spectroscopic processes (RT3), which depend in a subtle, unknown manner on electron correlation 
  • ultrastrong magnetic fields and under ultrahigh pressures (RT4), where molecules are squeezed and bind in new manners
  • chemical transformations (RT5)
  • multiphase systems (RT6)

In such situations, DFT and our intuition are pushed out of their comfort zones and are likely to fail, and new developments are vital.

For large systems, we will continue ongoing work on linear-scaling methods in LSDALTON [2]. Our recently developed code MRCHEM [7] for basis-set-free DFT calculations using multiwavelets is inherently linear scaling and will be extended to large systems, while also providing useful basis-set-limit results for RT3 and RT4.

For heavy atoms, relativity is crucial: two- and four-component methods will be implemented in LONDON and the efficient relativistic DFT code RESPECT [8] will be extended to the condensed phase. 

Semi-empirical methods

For even larger systems, semi-empirical methods such as density-functional tight binding (DFTB) will be implemented and reparameterized for our purposes.

 

 

[1] J. W. Furness et al. “Current density-functional the- ory using meta-generalized gradient exchange–correlation functionals”. J. Chem. Theory Comput. 11 (2015), 4169. [2] K. Aidas et al. “The Dalton quantum chemistry program system”. WIREs Comput. Mol. Sci. 4 (2014), 269. [3] F. Aquilante et al. “MOLCAS 7: The Next Generation”. J. Comput. Chem. 31 (2010), 224. [4] R. Bast et al. “DIRAC, a relativistic ab initio electronic structure program” (2015). [5] M. Ringholm et al. “A general open-ended response code using recursive programming”. J. Comp. Chem. 35 (2014), 622. [6] E. Tellgren et al. LONDON, a quantum-chemistry program for plane-wave/GTO hybrid basis sets and finite magnetic field calculations. 2016. [7] S. R. Jensen et al. “Magnetic Properties with Multiwavelets and DFT: the Complete Basis Set limit achieved”. Phys. Chem. Chem. Phys. (2016). in press, DOI: 10.1039/C6CP01294A. [8] M. Repisky et al. Respect, Relativistic Spectroscopy DFT Program. Version 3.3.0. 2013.