Psi4 is an open-source suite of ab initio quantum chemistry programs designed for efficient, high-accuracy simulations of a variety of molecular properties. We can routinely perform computations with more than 2500 basis functions running serially or on multi-core machines. With computationally demanding portions written in C++, Pybind11 exports many of the C++ classes into Python, and a flexible Python driver, Psi4 strives to be friendly to both users and developers.