  In order to analyze spectrum from the interstellar medium (ISM), spectrum of
the molecule of interest is recorded in a laboratory, and accurate rotational
and centrifugal distortion constants are derived. By using these constants, one
can calculate accurate partition function. However, in the same paper, where
these constants are derived, the partition function is calculated by using a
semi-empirical expression.
  We have looked into the details of this semi-empirical expression and
compared the values, obtained from it, with the accurate ones. As an example,
we have considered the case of Methanimine (CH$_2$NH) which is detected in a
number of cosmic objects. It is found that for the kinetic temperature $T >
120$ K, the semi-empirical expression gives large value as compared to the
accurate one. The deviation becomes about 25\% larger than the accurate one at
the kinetic temperature of 400 K.
