The PE library is an implementation of the polarizable embedding (PE) model which is a fully polarizable fragment-based quantum-classical approach, as described in the following works: Olsen, J. M. H., Aidas, K., and Kongsted, J., J. Chem. Theory Comput. 6, 2010, pp 3721–3734 (https://doi.org/10.1021/ct100380) Olsen, J .M. H. and Kongsted, J., Adv. Quantum Chem. 61, 2011, pp 107-143 (https://doi.org/10.1016/B978-0-12-386013-2.00003-6) Olsen, J. M. H., Ph. d. thesis (2012), Department of Physics, Chemistry and Pharmacy, University of Southern Denmark (https://doi.org/10.6084/m9.figshare.156852) New features in 1.2 include magnetic gradient using GIAOs as described in: Steinmann, C., Olsen, J. M. H., Kongsted, J., J. Chem. Theory Comput. 10, 2014, pp 981–988 (https://doi.org/10.1021/ct400880n) New features in 1.3 include molecular gradient and effective external fields as described in: List, N. H., Beerepoot, M. T. P., Olsen, J. M. H., Gao, B., Ruud, K., Jensen, H. J. Aa., Kongsted, J., J. Chem. Phys. 142, 2015, pp 034119 (https://doi.org/10.1063/1.4905909) List, N. H., Jensen, H. J. Aa., Kongsted, J., Phys. Chem. Chem. Phys. 18, 2016, pp 10070-10080 (https://doi.org/10.1039/C6CP00669H)