From Amber parameter set: atomic_ions.lib
See Li, Roberts, Chakravorty and Merz, JCTC, 9(6), pp 2733-2748, 2013

Residue   Atom     Ion 
 Name     Name    Charge
-------   ----    ------
 AG        AG      Ag+
 Ag        Ag      Ag2+
 AL        AL      Al3+
 BA        BA      Ba2+
 BR        BR      Br-
 Be        Be      Be2+
 CA        CA      Ca2+
 CD        CD      Cd2+
 CE        CE      Ce3+
 CL        CL      Cl-
 CO        CO      Co2+
 CR        CR      Cr3+
 CS        CS      Cs+
 CU        CU      Cu2+
 CU1       CU      Cu+
 Ce        Ce      Ce4+
 Cl-       Cl-     Cl-
 Cr        Cr      Cr2+
 Dy        Dy      Dy3+
 EU        EU      Eu2+
 EU3       EU3     Eu3+
 Er        Er      Er3+
 F         F       F-
 FE        FE      Fe3+
 FE2       FE2     Fe2+
 GD3       GD      GD3+
 H3O+      O       H3O+
 HE+       H       HE+
 HG        HG      Hg2+
 HZ+       H       HZ+
 Hf        Hf      Hf4+
 IN        IN      In3+
 IOD       I       I-
 K+        K+      K+
 LA        LA      La3+
 LI        LI      Li+
 LU        LU      Lu3+
 MG        MG      Mg2+
 MN        MN      Mn2+
 NA        NA      Na+
 NH4       N       NH4+
 NI        NI      Ni2+
 Na+       Na+     Na+
 Nd        Nd      Nd3+
 PB        PB      Pb2+
 PD        PD      Pd2+
 PR        PR      Pr3+
 PT        PT      Pt2+
 Pu        Pu      Pu4+
 RB        RB      Rb+
 Ra        Ra      Ra2+
 SM        SM      Sm3+
 Sm        Sm      Sm2+
 SR        SR      Sr2+
 Sn        Sn      Sn2+
 TB        TB      Tb3+
 TL        TL      Tl+
 Tl        TL      Tl3+
 Th        Th      Th4+
 Tm        Tm      Tm3+
 U4+       U       U4+
 V2+       V2+     V2+
 Y         Y       Y3+
 YB2       YB2     Yb2+
 ZN        ZN      Zn2+
 Zr        Zr      Zr4+