etc.). IQMol is developed by Andrew Gilbert (in coordination with Q-Chem) and is distributed as free open-source software. IQmol is written using the Qt libraries, enabling it to run on a range of platforms, including OS X, Widows, and Linux. It provides an intuitive environment to set up, run, and analyze Q-Chem calculations. It can also read and display a variety of file formats, including the widely available formatted checkpoint format. A complete, up-to-date list of features is published on the Q-Chem website and in the user manual. In addition, Q-Chem is interfaced with WebMO and is used as the computing engine in Spartan, or as a back-end to CHARMM, GROMACS, NAMD, and ChemShell. Other popular visualization programs such as Jmol and Molden can also be used. In 2018, Q-Chem established a partnership with BrianQC, produced by StreamNovation, Ltd., a new integral engine exploiting the computational power of GPUs. The BrianQC plug-in speeds up Q-Chem calculations by taking advantage of GPUs on mixed architectures, which is highly efficient for simulating large molecules and extended systems. BrianQC is the first GPU Quantum Chemistry software capable of calculating high angular momentum orbitals. Ground State Self-Consistent Field Methods Restricted, unrestricted, and restricted open-shell formulations Analytical first and second derivatives for geometry optimizations, harmonic frequency analysis, and ab initio molecular dynamics Efficient algorithms for fast convergence Variety of guess options (including MOM) Density functional theory Variety of local, GGA, mGGA, hybrid, double-hybrid, dispersion-corrected, range separated functionals (energies and analytic first and second derivatives) TDDFT and spin-flip-TDDFT formulations (energies, gradients, and frequencies) Constrained DFT Innovative algorithms for faster performance and reduced scaling of integral calculations, HF/DFT and many-body methods Dual basis Resolution of identity Cholesky decomposition of electron-repulsion integrals Continuous Fast Multipole Method (CFMM) Fast numerical integration of exchange-correlation with mrXC (multiresolution exchange-correlation) Linear-scaling HF-exchange method (LinK) Fourier transform Coulomb method (FTC) COLD PRISM and J-matrix engine Mixed-precision arithmetic for correlated methods 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) Extensions of DFT and many-body methods to treat core-level states and related spectroscopies ADC methods CIS, TDDFT, CIS(D), and SOS-CIS(D) methods for excited