AFAIR, this was the historical "feature" to enable TIP3P "solute" calculation. There is no molecular composition in psf file and thus we assumed that the solute molecule was at the first in the atomic ordering. Thus the first molecule, even if it is one of several identical molecules, is considered as the split one.
Now that this assumption is removed, solute may not be the first molecule in the system. I think we can safely remvoe this "feature", but I need to dig the past mails and mercurial logs first.
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AFAIR, this was the historical "feature" to enable TIP3P "solute" calculation. There is no molecular composition in psf file and thus we assumed that the solute molecule was at the first in the atomic ordering. Thus the first molecule, even if it is one of several identical molecules, is considered as the split one.
Now that this assumption is removed, solute may not be the first molecule in the system. I think we can safely remvoe this "feature", but I need to dig the past mails and mercurial logs first.