Research: Carter Group Software
PROFESS (PRinceton Orbital-Free Electronic Structure Software): An orbital-free density functional theory program for condensed matter computations.
MOLCAS-embed
Goal:
Provide an improved description of electron exchange and correlation in a local region of condensed matter via an embedded cluster method. Currently, we are focusing on molecule/metal surface interactions. The cluster of atoms close to the molecule is treated with multi-reference singles and doubles CI, while the background atoms are described by density functional theory. A DFT-based embedding potential describes the effect of the background atoms on the cluster.
Implementation in MOLCAS:
· Construct the embedding potential from orbital-free DFT.
· SEWARD module: read in embedding potential and apply as an external potential to the cluster.
· RASSCF module: optimize the cluster molecular orbitals in the presence of the embedding potential.
· GUGA/MOTRA/MRCI modules: get an embedded MRSDCI wavefunction
· Use the embedded wavefunction to find the embedding total energy.
References
P. Huang and E.A. Carter, J. Chem. Phys., 125,084102 (2006).
Linear Scaling MRSDCI/Reduced Scaling MRACPF
Goal:
A Linear Scaling MRSDCI alogorithm:
We have achieved linear scaling within multireference singly and doubly excited configuration interaction (MRSDCI) theory. A mulitreference treatment is often necessary for reactions involving bond-breaking, diradicals and transition metals.
Unfortunately, a direct MRSDCI code scales typically as O(N6) and severely limits the size of the system that can be investigated. Nevertheless, massive reduction in computational overhead is possible by using localized orbitals to span the occupied and virtual space.1,2 By employing local truncation schemes together with integral screening, the scaling of the method can be reduced to linear.
Scheme
Our MRSDCI and MRACPF codes3-5 have been embedded as BREWIN_CI3-5 and TIGER_CI into MOLCAS and calls the following modules to generate required quantities
(A) SEWARD generates the two electron integrals required for the following iteration cycles in SCF/RASSCF and also in BREWIN_CI for the generation of the Hamiltonian matrix elements.
(B) SCF/RASSCF generates the wavefunction
(C) For local MRSDCI calculations, localized orbitals are generated via LOCALISATION
(D) Our plug-in BREWIN_CI performs a MRSDCI or MRACPF calculation based on the symmetric group graphical approach (SGGA).7
References:
1. S. Saebo, P. Pulay,J. Chem. Phys. 86, 914 (1987).
2. S. Saebo, P. Pulay, Ann. Rev. Phys. 44, 213 (1993)
3. D. Walter and E.A. Carter. Chem. Phys. Lett. 346 , 177 (2001),
4. D. Walter, A. Venkatnathan and E. A. Carter, J. Chem. Phys. 118, 8127 (2003)
5. A. Venkatnathan, A. B. Szilva, D. Walter, R. J. Gdanitz and E. A. Carter, J. Chem. Phys. 120, 1693 (2004)
6. T. S. Chwee, A. B. Szilva, R. Lindh, and E. A. Carter, J. Chem. Phys. 128, 224106 (2008).
7. W. Duch , J. Karwowski, Comput. Phys. Rep. 2, 95 (1985).
GAMESS - AIDFT+U (ab initio DFT+U)
Goal:
Evaluate ab initio the Coulomb and exchange parameters for DFT+U calculations. DFT+U theory is based on DFT, but the intra-atomic Coulomb and exchange interactions of localized valence electrons are effectively treated at the Hartree-Fock level of theory. DFT+U theory can correct the self-interaction errors in DFT, given the average Coulomb (U) and exchange (J) interactions of these localized valence electrons as input. To obtain these two parameters, previously researchers either empirically fitted them or performed constrained DFT calculations. We recently proposed instead to evaluate the U and J using unrestricted Hartree-Fock calculations on electrostatically embedded clusters.
Implementation in GAMESS:
The method used to evaluate these parameters is based on unrestricted Hartree-Fock calculations. A few subroutines were modified to get GAMESS to calculate U and J.
The modifications to each subroutine are:
gamess.src: check input files and variables;
prppop.src: get Mulliken populations for calculating U and J;
int2a.src: calculate the onsite two-electron integrals in the basis of the atomic orbitals;
rhfuhf.src: extensive modifications to calculate U and J;
scflib.src: calculate the Coulomb and exchange integrals through building the Fock matrix with direct SCF methods.
References:
N. J. Mosey and E. A. Carter, Physical Review B, 76, 155123 (2007)
N. J. Mosey, P. Liao, and E. A. Carter, Journal of Chemical Physics, 129, 014103 (2008)