To start, we'll neglect the bosonic degrees of freedom (the displacement of the atoms themselves) and look at only the fermionic DoF (the electrons).
Water has 8 electrons (which QC can treat exactly without any extra work) in a number of orbital. In general we need 2 qubits per orbital.
Most QC demonstrations so far were performed using so-called minimal basis sets, which have a small number of orbitals and thus give inaccurate results. A better approach would be to take a large orbital basis, do a classical relatively expensive Hartree-Fock calculation then use the orbitals from that to do the QC. This technique when done on classical computers is called MRCI (multi-reference configuration interaction) and is the gold standard in Quantum Chemistry.
So, provided we can pay the cost of doing a large orbital HF calculation (and we can do that for fairly large molecules), we can get pretty good result using n electrons in n orbitals MRCI. So production electronic calculations of water molecules would take about 16 qubits per molecule.
The more frustrating problem is that the number of electronic interaction terms is N^4 the number of orbitals so that we would very rapidly need extremely deep circuits which are not feasible without error correction (which involve using like 8 actual qubits for every calculation qubit). There are proposal to use plane wave basis sets (N^2 interactions) but then we need many more orbitals and thus many more qubits.
We are in practice very far from QC having a significant impact on real-life quantum chemistry. It's not at all clear that we'll ever be able to do QC on a molecule the size of a typical drug, let alone a protein.
because Qbits are just random numbers. They want to make u believe that Qbits represent a range of values, which is true in theory, but in reality it's just a plain old single random number. So all QC calculation are basically random + random * random - random = ... random ofc. I think google it's only QC application was a program that generates some random hash. Other than that, QC hasnt done anything else, nor do i believe it can.
really? How come reality keeps agreeing with me then? Where are these amazing quantum mechanics applications? U really believe in super position?? really? How long have they been working on QC? 50 years now? it never seems to work or be ready.. i wonder why.
I have no idea why you think reality agrees with you. I “believe” in superposition, entanglement, and the like because I’ve tested it myself in a lab with a six qubit quantum computer I built.
