Nomenclature of dH/dlamda in free energy .xvg file

GROMACS version: 2019.3
GROMACS modification: No
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Dear users,
I am a little bit confused with the nomenclature of dH/dlamda in free energy .xvg file.

@ yaxis label “dH/d\xl\f{} and \xD\f{}H (kJ/mol [\xl\f{}]\S-1\N)”

I know the y axis should be dH/dlambda, but I am still curious to know the meaning of
“\xl”, “f{}”, and “\xD”, “f{}H” in this comment line

XmGrace formatting. Plot the file in XmGrace and you will see what they mean.

Thanks! It works. I find they are called typesetting string.