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You can use all the end entries in the Select... Chemistry menu also in the command line. These symbols are in the element submenu. In cases where there is ambiguity, for example H appears both under element and under IDATM type, you can disambiguate like

select element.H

It will work to use element.Mg and element.Ca ... it's just more typing.

Hum that does not seem to work oddly. "Select element.Ca" gives me a "mangled atom specifier" (that why I asked you in the first place). Same thing for Mg btw (I originally though there was some ambiguity between Calcium and carbon-alpha naming).

Whoops, my bad. Actually Mg and Ca are not at the top level of "Select... Chemistry... element" but in further submenus. You could use "Md-Ni.Mg" and "Be-Cl.Ca" but I don't think there is a need to use anything longer than Mg and Ca. Ca will not conflict with atom name because the command-line syntax requires @ to indicate atom name. "@ca" are atoms named CA, "@c" are atoms named C, "Ca" are calcium ions/atoms, "C" are carbon atoms. The capitalization is not important for atom names, but it IS important for the things that are also menu entries and should match what is shown in the menu. The potential for ambiguity with element symbols is with the entries under "Select... Chemistry... IDATM type" but in fact H is the only one I can think of that appears in both places. Best, Elaine