bug fix to nonequilibrium MM2015 formulation

[oalcabes]

Purpose

To-do

Content

---

• I have read and checked the items on the review checklist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.89%. Comparing base (9582b2d) to head (992c2c3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files          41       41           
  Lines        1480     1480           
=======================================
  Hits         1434     1434           
  Misses         46       46           

Components	Coverage Δ
src	98.77% <100.00%> (ø)
ext	69.79% <ø> (ø)

[trontrytel]

Thank you!