In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already
includes a large number of new developments realized during the transition from
the commercial MOLCAS product to the open-source platform. The paper initially
describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features
of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density
matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations
include an array of additional options and functionalities. The paper proceeds and
describes developments related to explorations of potential energy surfaces. Here
we present methods for the optimization of conical intersections, the simulation of
adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes
features unique to simulations of spectroscopic and magnetic phenomena such as
the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper
describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option
using frozen-density embedding theory and new electronic and muonic basis sets
