[Diff since v0.6.19](https://github.com/JuliaMolSim/DFTK.jl/compare/v0.6.19...v0.6.20) **Merged pull requests:** - Virial interface for AtomsCalculator (#959) (@CedricTravelletti) - Calculator State (#961) (@CedricTravelletti) - Add a preference option for DFTK threading (#972) (@antoine-levitt) - Ignore optim_res when saving scfres (#973) (@antoine-levitt) - Improve performance of stress calculations (#978) (@mfherbst) - Use the Simpson rule for integration of local part of PSPs (#982) (@abussy) - fix typo (#983) (@chillenb) - Update chat url to zulip (#984) (@mfherbst) - Updating TestItemRunner (#989) (@epolack) - Update Hartree potential notation in introductory docs (#990) (@niklasschmitz) **Closed issues:** - Stresses need an unreasonable amount of memory and runtime (#893) - Incompatible with TestItemRunner 1.0 (#988)