Q-Chem is a general-purpose electronic structure package featuring a variety of established and new methods implemented using algorithms that enable fast calculations of large systems on various computer architectures. It is compatible with laptops, regular lab workstations, midsize clusters, and cloud computing and uses density functional and/or wave-function based approaches. It offers an integrated graphical interface and input generator; a large selection of functionals and correlation methods, including methods for electronically excited states and open-shell systems; solvation models; and wave-function analysis tools. In addition to serving the computational chemistry community, Q-Chem also provides a versatile code development platform.

History

Q-Chem software is maintained and distributed by Q-Chem, Inc.,).

The first lines of the Q-Chem code were written by Peter Gill, at that time a postdoc of Pople, during a winter vacation (December 1992) in Australia. Gill was soon joined by Benny Johnson (a Pople graduate student) and Carlos Gonzalez (another Pople postdoc), but the latter left the company shortly thereafter. In mid-1993, Martin Head-Gordon, formerly a Pople student, but at that time on the Berkeley tenure track, joined the growing team of academic developers. in which about 350,000 calculations were performed daily on the IBM World Community Grid.

thumb|Figure 3. Statistics of Q-Chem developer activity since 2006. Top chart: Total number of code commits (height of bars) and number of developers contributing (color of bar) by month. Bottom chart: Growth of developer base, showing existing and new developers each month. A steady growth of the developer base can be seen. The inset depicts the total number of commits by the 50 most-prolific developers, showing contributions by full-time team (> 2000 commits), the core developer team (500–2000 commits), and non-core developers (< 500 commits).

Innovative algorithms and new approaches to electronic structure have been enabling cutting-edge scientific discoveries. This transition, from in-house code to major electronic structure engine, has become possible due to contributions from numerous scientific collaborators; the Q-Chem business model encourages broad developer participation. Q-Chem defines its genre as open-teamware:

Post Hartree–Fock methods

  • MP2 (including RI-MP2, energies and analytic gradients)
  • SCS, SOS-MP2, and OO-MP2
  • CCD, QCISD, CCSD, OOCCD, VOOCCD
  • (T), (2), (dT), and (fT) corrections
  • EOM-XX-CCSD methods for open-shell and electronically excited species (XX=EE, SF, IP, EA, DIP, DEA, 2SF; energies, properties, and gradients for most methods), including complex-valued variants for treating resonances (states metastable with respect to electron detachment)
  • ADC methods
  • CIS, TDDFT, CIS(D), and SOS-CIS(D) methods for excited states
  • Variety of implicit solvent models
  • Wave-function analysis tools enabled by libwfa developed by Felix Plasser and co-workers

QM/MM and QM/EFP methods for extended systems

  • Janus QM/MM interface
  • YinYang Atom model without linked atoms
  • ONIOM model
  • EFP method (including library of effective fragments, EFP interface with CC/EOM, DFT/TDDFT, and other methods)

Version history

Beginning with Q-Chem 2.0 only major releases versions are shown.

  • Q-Chem 1.0: March 1997
  • Q-Chem 1.1: 1997
  • Q-Chem 1.2 1998
  • Q-Chem 2.0: 2000