Internal energy `u0` in the QM9 dataset


I am new to DeepChem and have recently been experimenting with a new machine learning method for molecular property prediction using the QM9 dataset as a benchmark. Initially, I assumed that u0 represents the ground electronic-state energy plus the zero-point energy, and as such, it should be negative for stable molecules.

However, upon examining the distribution of the internal energy at 0 Kelvin ( u0 ) in the dataset, I noticed that it encompasses both positive and negative values, falling within the approximate range of [-3.54, 6.620]. This discovery has prompted me to wonder how exactly u0 in the QM9 dataset was calculated. Despite my efforts in researching this, I have yet to find a satisfactory answer.

Could someone here kindly provide information on how u0 is calculated or perhaps direct me to a reference that explains this calculation in detail? Your assistance would be greatly appreciated. Thank you very much for your time in advance!