How to represent a 0-valid boolean formula? I read in these two papers http://www.ccs.neu.edu/home/lieber/courses/csg260/f06/materials/papers/max-sat/p216-schaefer.pdf and http://people.csail.mit.edu/madhu/papers/noneed/fullbook.ps that if we have a boolean formula that is $0-valid$ then (of course) SAT problem is in $\mathcal{P}$ but finding a solution with maximum true literals is $\mathcal{NP}-$hard. 

N.B. As defined in the previous papers, a $0-valid$ boolean formula $f$ is a boolean formula $f: \{0, 1\}^n\rightarrow\{0, 1\}$ that satisfies $f(0, \dotsc, 0)=1$.

My question is:

Can I represent a general $0-valid$ boolean formula on the variables $x=\left(x_1, \dotsc, x_n\right)$ by the following one:

$f(x)=\bigwedge\limits_{i=1}^{L}\bigvee\limits_{i\in\mathcal{S}_l}\neg\;x_i$?

Where $L$ is the number of clauses and $\mathcal{S}_l\;\forall\;l\in\{1,\dotsc, L\}$ is a subset of $\{1, \dotsc, n\}$.
The constraints of a $0$-valid formula can be more general. All we know about them is that they are satisfied if all variables are $0$. For example, a constraint could be $x = y$ (i.e. $(x \land y) \lor (\lnot x \land \lnot y)$). Your form is not general enough, since it only considers constraints of the form $\lnot x_1 \lor \dots \lor \lnot x_\ell$.

While your formulation doesn't capture the problem of determining the most positive solution for a $0$-valid CSP, it is equivalent to an even more classical problem, set cover. In particular, your problem is NP-complete.