# Thermally-Assisted-Occupation Density Functional Theory (TAO-DFT)

The thermally-assisted-occuptation DFT (TAO-DFT) method, now available in Q-Chem, provides accurate descriptions of the ground states of strongly correlated systems. It uses partial orbital occupations, which are defined by a Fermi-Dirac distribution and controlled by a fictitious temperature $$\theta$$, to account for multireference character.

*Occupation numbers of the $$3\sigma_g$$ orbital for N$$_2$$ compared to MRCI values. *

*Potential energy surface for the torision of twisted ethylene. *

• Improved DFT description of the ground states of strongly correlated systems

• Similar computational cost to traditional KS-DFT for energies and analytical nuclear gradients

• No pre-determined active space required

• Can be used with existing XC functionals (e.g. LDA or GGA)

• Allows simulation of very large, strongly correlated polyradical systems on the nanoscale

• Can be combined with ab initio molecular dynamics approaches (TAO-AIMD)