We investigate the formation and early evolution of star clusters assuming that they form from a turbulent starless clump of given mass bounded inside a parent self-gravitating molecular cloud characterized by a particular mass surface density. As a first step we assume instantaneous star cluster formation and gas expulsion. We draw our initial conditions from observed properties of starless clumps. We follow the early evolution of the clusters up to 20 Myr, investigating effects of different star formation efficiencies, primordial binary fractions and eccentricities and primordial mass segregation levels. We investigate clumps with initial masses of $M_{rm cl}=3000:{rm M}_odot$ embedded in ambient cloud environments with mass surface densities, $Sigma_{rm cloud}=0.1$ and $1:{rm g:cm^{-2}}$. We show that these models of fast star cluster formation result, in the fiducial case, in clusters that expand rapidly, even considering only the bound members. Clusters formed from higher $Sigma_{rm cloud}$ environments tend to expand more quickly, so are soon larger than clusters born from lower $Sigma_{rm cloud}$ conditions. To form a young cluster of a given age, stellar mass and mass surface density, these models need to assume a parent molecular clump that is many times denser, which is unrealistic compared to observed systems. We also show that in these models the initial binary properties are only slightly modified by interactions, meaning that binary properties, e.g., at 20 Myr, are very similar to those at birth. With this study we set up the basis of future work where we will investigate more realistic models of star formation compared to this instantaneous, baseline case.
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