I know this question looks very similar to many others but I actually think it is not the same, because I was looking for a possible answer for hours. The question is about the first step in the pp chain (i.e. the following reaction)
$$\rm^{1}H+{}^{1}H \to {}^{2}H+e^{+}+\nu_{e}$$ I wanted to know if the following reasoning is correct. In order to point out easier the possible mistakes I separated it in steps: Context: Imagine the two initial $\rm^{1}H$ has zero kinetic energy (I know this is difficult but I suppose there are in some point of the potential such that this is possible), because I am not really interested in kinetic energy (at least before the fusion takes place).
The initial sistem before the fusion has an energy equal to $2mc^2$ where $m$ is the mass of the $\rm^{1}H$ (when it is "free" or alone).
After the process takes place the two $\rm^{1}H$ stay together (forming $\rm^{2}H$) and due to this a negative potential energy appears i.e. binding energy (like when two bodies come closer in a gravitational field).
As $E_{\rm^{2}H}=m_{\rm^{2}H}c^2$ the previous fall in energy of this part of the products is readed as a reduction in mass (because even taking into account the energies due to the masses of the positron and the neutrino is there a fall) and is called "mass defect".
To balance energy before and after the fusion takes place an equal amount of POSITIVE released energy has to be created (usually called $Q$).
This energy (which is 0.42 Mev) is usually the kinetic energy associated with the products of the reaction (and sometimes in other reactions even with radiation).
I really appreciate an answer, because many books are not absolutely clear about this point.