# psi4

## Quantum Chemical Program Suite

Description

PSI4 is an ab-initio quantum chemistry program. It is especially designed to accurately compute properties of small to medium molecules using highly correlated techniques. PSI4 is the parallelized successor of PSI3 and includes many state-of-the-art theoretical methods. It can compute energies, gradients and hessians for the following methods: * Restricted Hartree-Fock (RHF)

It can compute energies and gradients for the following methods:
* Restricted, unrestricted and general restricted open shell Hartree-Fock

(RHF/ROHF)

* Restricted, unrestricted and general restricted open shell

Densitry-Functional Theory, including density-fitting (DF-DFT)

* Density Cumulant Functional Theory (DCFT)

* Density-fitted Moeller-Plesset perturbation theory (DF-MP2)

* Density-fitted Orbital-Optimized MP2 theory (DF-OMP2)

* (Orbital-Optimized) MP3 theory (OMP3/MP3)

* Coupled-cluster singles doubles (CCSD)

* Density-fitted coupled-cluster singles doubles (DF-CCSD) and with

perturbative triples (DF-CCSD(T))

* Second-order approximate coupled-cluster singles doubles (CC2)

* Equation-of-motion coupled-cluster singles doubles (EOM-CCSD)

Additionally, it can compute energies for the following methods:
* Spin-component scaled MP2 theory (SCS-MP2)

* Fourth order Moeller-Plesset perturbation theory (MP4)

* Density-fitted symmetry-adapted perturbation theory (DF-SAPT)

* Density-fitted complete active space SCF (DF-CASSCF)

* Configuration-interaction singles doubles (CISD)

* Full configuration-interaction (FCI)

* Closed-shell Density-fitted coupled-cluster singles doubles (DF-CCSD)

* Closed-shell Density-fitted Coupled-cluster singles doubles with

perturbative triples (DF-CCSD(T))

* Second/third-order approximate coupled-cluster singles doubles (CC2/CC3)

* Mukherjee Multireference coupled-cluster singles doubles theory (mk-MRCCSD)

* Mukherjee Multireference coupled-cluster singles doubles with perturbative

triples theory (mk-MRCCSD(T))

* Second order algebraic-diagrammatic construction theory (ADC(2))

* Quadratic configuration interaction singles doubles (QCISD)

* Quadratic configuration interaction singles doubles with perturbative

triples (QCISD(T))

* Density Matrix Renormalization Group SCF (DMRG-SCF), CASPT2 (DMRG-CASPT2)

and CI (DMRG-CI)

Further features include:
* Flexible, modular and customizable input format via Python

* Excited state calculations with the EOM-CC2/CC3, EOM-CCSD, ADC(2), MRCI and

mk-MRCC methods

* Utilization of molecular point-group symmetry to increase efficiency

* Internal coordinate geometry optimizer

* Harmonic frequencies calculations (via finite differences)

* Potential surface scans

* Counterpoise correction

* One-electron properties like dipole/quadrupole moments, transition dipole

moments, natural orbitals occupations or electrostatic potential

* Composite methods like complete basis set extrapolation or G2/G3

* Scalar-relativistic corrections via two-component approach (X2C)

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