If you require COMSOL Multiphysics for professional or academic work, the recommended approach is to avoid the "free download" rabbit hole and utilize official channels:

Websites claiming to offer COMSOL 3.5a for free may contain:

That said, I can write an informative, responsible article that:

to check for available downloads associated with your subscription. Contact Support

Version 3.5a was designed for Windows XP/Vista and older Linux kernels. It may not even run on modern Windows 10/11 or macOS without significant troubleshooting.

Version 3.5a was lauded for its seamless integration with MATLAB via LiveLink. Some legacy workflows and custom scripts were built specifically around the API architecture available in 3.5a. Updating these scripts for modern versions can be time-consuming, leading some to prefer running the older software.

: Version 3.5a was designed for much older operating systems (like Windows XP or Vista) and may not run correctly on modern hardware or OS versions without significant troubleshooting.

Today, you have three clean options: