Project Summary This proposal responds to PA-14-156, Extended Development, Hardening and Dissemination of Technologies in Biomedical Computing, Informatics and Big Data Science. The goal of this program is to support the continued development, maintenance, testing and evaluation of existing software, matching our proposal. Amber is a very popular software package, used by academic and industry institutions, for simulating the structural, thermodynamic and kinetic properties of molecular systems. The free version of Amber has been downloaded by over 10,000 unique users. There are 3,500 citations to the widely adopted Amber ff99SB protein force field, developed at Stony Brook by PI Carlos Simmerling. The Simmerling group is one of the six academic labs responsible for leading the Amber software maintenance and development effort. Computational molecular dynamics simulations using Amber and other software packages have become essential counterparts to experimental research for understanding the mechanisms of biomolecules, and for discovering drugs to inhibit them. The popular virtual screening program called DOCK interfaces directly with Amber, providing improvements to this widely used program for drug discovery. Other widely used programs also interface with Amber. Productivity in our prior funding period was excellent, leading to articles published in PNAS (2), JACS (3), JCTC (4) and other high-impact journals, with others currently in review. Several new versions of Amber were released. We propose here new developments for Amber, addressing two of the most pressing needs of the field. Aim 1, Solvation: We will incorporate improved solvation models: (a) the SEA semi- explicit water model and (b) a fast but reasonably accurate Generalized Born implicit water model. Aim 2, Sampling: We will add fast and targeted sampling methods: (a) variants of the general tools Hamiltonian exchange and thermal Replica Exchange Molecular Dynamics, (b) the very fast Kinetic-Loop-Closure and Constrained-Loop-Closure methods for sampling loop conformations. Following our practice over the past 20 years, we will openly share our results, parameters and methods, and incorporate them into the Amber production code for distribution with fully documented source code, along with full documentation, test cases, and tutorials. New versions follow an annual release schedule.