On the correct spin lattice for the spin-gapped magnetic solid NH4CuPO4·H2O

NH4CuPO4·H2O is a spin-gapped compound that has been described in terms of an isolated antiferromagnetic spin dimer model. To explore the origin of this spin gap, we examined the spin exchange interactions of NH4CuPO4·H2O by performing qualitative spin dimer analysis based on extended Hückel tight binding calculations and also by carrying out quantitative mapping analysis based on first principles density functional theory electronic band structure calculations. Our study indicates that, to a first approximation, the magnetic properties of NH4CuPO4·H2O should be described by an antiferromagnetic and ferromagnetic alternating chain.