You could try replacing "ln" with "log"

Many languages (C, matlab) use "log" for the natural log, and log10 for base10.  Verilog-A is one of the few exceptions that use "ln" for natural log (and "log" is base10).  You should probably check that log gives you the natural log in that simulator's model file language -- the lazy man's approach is to check that "log10" is defined, and assume that implies that "log" is natural log.