We say that a function $\varphi:X^{\rm an}\to{\mathbb{R}}$ is a model function if there exists $d\in{\mathbb{N}}_{>0}$ and $\|\ \|$ an algebraic metric on $\mathcal{O}_{X^{\rm an}}$ such that $\varphi=-\frac{1}{d}\log\|1\|$. If we can take $d=1$, we say that $\varphi$ is a ${\mathbb{Z}}$-model function. The set of model functions on $X$ is denoted by $\mathcal{D}(X)$.