  Galactic interactions and subsequent mergers are a paramount channel for
galaxy evolution. In this work, we use the data from 236 star forming CALIFA
galaxies with integrated molecular gas observations in their central region
(approximately within an effective radius) -- from the APEX millimeter
telescope and the CARMA millimeter telescope array. This sample includes
isolated (126 galaxies) and interacting galaxies in different merging stages
(110 galaxies; from pairs, merging and post-merger galaxies). We show that the
impact of interactions and mergers in the center of galaxies is revealed as an
increase in the fraction of molecular gas (compared to isolated galaxies).
Furthermore, our results suggest that the change in star formation efficiency
is the main driver for both an enhancement and/or suppression of the central
star formation -- except in merging galaxies where the enhanced star formation
appears to be driven by an increase of molecular gas. We suggest that
gravitational torques due to the interaction and subsequent merger transport
cold molecular gas inwards, increasing the gas fraction without necessarily
increasing star formation.
