QuacPac 1.3.1 could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this software available. We recommend checking the downloaded files with any free antivirus. This program is an intellectual property of OpenEye Scientific Software, Inc. The software lies within Education Tools, more precisely Science Tools. The most popular version among QuacPac users is 1.3.
From the developer:
QUACPAC offers everything necessary to do charges well. As the chemistry of molecular interactions is a matter of shape and electrostatics, accurate or at least consistent charge representation is critical in drug design. However even the best charge models are of limited value if protonation states are wrong.
QUACPAC provides pKa and tautomer enumeration in order to get correct protonation states.
You may want to check out more software, such as Maxima and Minima, IUCLID5 or ECoord, which might be related to QuacPac.