  Many-body simulations of quantum systems is an active field of research that
involves many different methods targeting various computing platforms. Many
methods commonly employed, particularly coupled cluster methods, have been
adapted to leverage the latest advances in modern high-performance
computing.Selected configuration interaction (sCI) methods have seen extensive
usage and development in recent years. However development of sCI methods
targeting massively parallel resources has been explored only in a few research
works. In this work, we present a parallel, distributed memory implementation
of the adaptive sampling configuration interaction approach (ASCI) for sCI. In
particular, we will address key concerns pertaining to the parallelization of
the determinant search and selection, Hamiltonian formation, and the
variational eigenvalue calculation for the ASCI method. Load balancing in the
search step is achieved through the application of memory-efficient determinant
constraints originally developed for the ASCI-PT2 method. Presented benchmarks
demonstrate parallel efficiency exceeding 95\% for the variational ASCI
calculation of Cr$_2$ (24e,30o) with $10^6, 10^7$, and $3*10^8$ variational
determinants up to 16,384 CPUs. To the best of the authors' knowledge, this is
the largest variational ASCI calculation to date.
