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The EXCHANGE and CORRELATION *$rem* variables (Chapter 4) allow the user to indicate that the exchange-correlation density functional will be user-defined. The user defined exchange-correlation is to be entered in the *$xc_functional* part of the input. The format is:

$xc_functional X exchange_symbol coefficient X exchange_symbol coefficient ... C correlation_symbol coefficient C correlation_symbol coefficient ... K coefficient $end

**Note: **Coefficients are real numbers.