First-principles study of ground- and excited-state properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>MgO</mml:mi></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>ZnO</mml:mi></mml:mrow></mml:math>, and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>CdO</mml:mi></mml:mrow></mml:math>polymorphs

An ab initio pseudopotential method based on density functional theory, generalized gradient corrections to exchange and correlation, and projector-augmented waves is used to investigate structural, energetical, electronic, and optical properties of $\mathrm{MgO}$, $\mathrm{ZnO}$, and $\mathrm{CdO}$ in rocksalt, cesium chloride, zinc blende, and wurtzite structure. In the case of $\mathrm{MgO}$ we also examine the nickel arsenide structure and a graphitic phase. The stability of the ground-state phases rocksalt $(\mathrm{MgO},\mathrm{CdO})$ and wurtzite $(\mathrm{ZnO})$ against hydrostatic pressure and biaxial strain is studied. We also present the band structures of all polymorphs as well as the accompanying dielectric functions. We discuss the physical reasons for the anomalous chemical trend of the ground-state geometry and the fundamental gap with the size of the group-II cation in the oxide. The role of the shallow $\mathrm{Zn}3d$ and $\mathrm{Cd}4d$ electrons is critically examined.