There are two possible other citations for CLM use directly in RegCM.
I believe the current RegCM version uses CLM3.5 with plans to incorporate CLM4
For CLM3.0 use in RegCM cite:
Steiner AL, Pal JS, Giorgi F, Dickinson RE, Chameides WL (2005) Coupling of the Common Land Model (CLM0) to a regional climate model (RegCM). Theor Appl Climatol 82(3–4):225–243. doi:10.1007/s00704-005-0132-5
For CLM3.5 use in RegCM cite:
Tawfik, A. B., and A. L. Steiner (2011), The role of soil ice in land‐atmosphere coupling over the United States: A soil moisture–precipitation winter feedback mechanism, J. Geophys. Res., 116, D02113, doi:10.1029/2010JD014333
-Ahmed