An extensive series of 128 halogen-bonded complexes formed by trimethylphosphine oxide and various F-, Cl-, Br-, I- and At-containing molecules, ranging in energy from 0 to 124 kJ/mol, is studied by DFT calculations in vacuum. The results reveal correlations between R–X···O=PMe₃ halogen bond energy ∆E, X···O distance r, halogen’s σ-hole size, QTAIM parameters at halogen bond critical point and changes of spectroscopic parameters of phosphine oxide upon complexation, such as ³¹P NMR chemical shift, ∆δP, and P=O stretching frequency, ∆ν. Some of the correlations are halogen-specific, i.e., different for F, Cl, Br, I and At, such as ∆E(r), while others are general, i.e., fulfilled for the whole set of complexes at once, such as ∆E(∆δP). The proposed correlations could be used to estimate the halogen bond properties in disordered media (liquids, solutions, polymers, glasses) from the corresponding NMR and IR spectra.