...$\{\mathbf{p},\mathbf{q}\}$

$\{\mathbf{p},\mathbf{q}\}$ stands for the point in the 6N dimensional phase space, and $d\{\mathbf{p},\mathbf{q}\}$ for the 6N-dimensional volume element

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...method

The primary purpose of the DATA statement is to give names to constants; instead of referring to $\pi$ as 3.141592653589793 at every appearance, the variable PI can be given that value with a DATA statement and used instead of the longer form of the constant. This also simplifies modifying the program, should the value of $\pi$ change.
FORTRAN manual for Xerox Computers

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...success.

unless something else goes wrong...

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