Perple_X input files involve numerous linear functions of pressure and temperature (e.g.,
Margules parameters, van Laar size terms, DQF corrections, and enthalpy of ordering) of the
general form:

f = c_0 + c_T * T + c_P * P

Prior to the 6.7.3 version of Perple_X, when the coefficients were read, all three
coefficients were entered as in:

c_0 c_T c_P               [DQF corrections in the thermodynamic data file make definitions]
DQF(A) =  c_0 c_T c_P     [DQF corrections in the solution model file]
W(A B) c_0 c_T c_P        [Margules parameters in the solution model file]
a(A)   c_0 c_T c_P        [van Laar size terms in the solution model file]
... enthalpy = c_0        [enthalpy of ordering in the solution model file]

The 6.7.3 version of Perple_X reads these formats, but allows a more flexible format in
which c_T and c_P can be omitted if zero and tags are used to identify the coefficients
(meaning that users no longer need to remember the order in which the c_T and c_P 
coefficients are entered; additional tags (e.g., '=' and '*') are also allowed. The general
format is (all on one line):

[name_tag] [=] c_0 [ [+/-] c_i [*] coefficient_tag]

where brackets indicate optional components and: +/- indicates a plus or minus sign; the
name_tag is prescribed for Margules parameters (e.g., W(A B)), van Laar size terms (e.g., 
a(A) and DQF corrections in the solution model file (e.g., DQF(A)); the = tag is mandatory
for enthalpy of ordering expressions; and the coefficient c_0 is mandatory in all input,
but, of course, may be zero; the coefficient tags must be < 9 characters and begin with an
upper or lower case T for the temperature coefficient. The coefficients may be rational
numbers (a) or fractions (a/b). 

The text interpreter is fairly robust, but to be safe include a blank space before +/- 
signs and after name and coefficient tags.

Examples of valid DQF function expressions in thermodynamic data file make definitions:

12d3 -0.14 1.
    DQF(J/mol) = 0
DQF 1/2 -1*T_K
fudge_factor = 3 + 4*Pbar

Prior to the 6.7.3 the enthalpy of ordering term in solution models was assumed to be a 
constant (c_0), it is now allowed linear pressure-temperature dependence.