I did an computational chemistry experiment using ORCA and Avogadro app to find information on small molecules (e.g H20, CH4, NH3, HF). If we use ab-initio quantum chemistry method to calculate large molecules (such as proteins) , the computational cost will be to big to calculate. So are there any methods to calculate large molecules? (answer should relate to Molecular mechanics and Force field)

Please answer them as if they were essay questions thank you