Molecular interpretation of X (3960) as D-s(+) D-s(-) state

We study Ds+Ds- and D (D) over bar states assuming that they are hadronic molecules with J(PC) = 0(++) quantum number. We use two-point QCD sum rule formalism and extract the mass and decay constant values of these states. We take into account contributions of various quark, gluon, and mixed vacuum condensates up to dimension eight. The extracted mass and decay constant values of D (D ) over bar and Ds+Ds- states read as M-D (D) over bar = 3795(-82)(+85 )MeV, f(D (D) over bar) = 1.70(-0.29)(+0.33) x 10(-2 )GeV(5), and MDs+Ds- = 3983(-88)(+93) MeV, f(Ds+Ds-) = 2.52(-0.54)(+0.64) +/- 0. 4 x 10(-2) GeV5, respectively. The predicted mass of Ds+Ds- state is in good agreement with the recent LHCb observation and supports quantum number and molecular picture assignments. A possible observation of D (D) over bar state would help for establishing the lowest four-quark state in charmonium sector.