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The ground state and first singlet excited states of ethylene,so-called N and V states respectively,are studied by means of modern valence bond methods.It is found that extremely compact wave functions,made of three VB structures for the N state and four structures for the V state,provide an N-V transition energy of 8.01 eV,in good agreement with experiment(7.88 eV for the N-V transition energy estimated from experiments).Further improvement to 7.96/7.93 eV is achieved at the variational and diffusion Monte Carlo(MC)levels respectively,VMC/DMC,using a Jastrow factor coupled with same compact VB wave function.Furthermore,the measure of the spatial extension of the V state wave function,19.14 a0 2,is in the range of accepted values obtained by large-scale state-of-the-art molecular orbital-based methods.The σ response to the fluctuations of the electrons in the V state,known to be a crucial feature of the V state,is taken into account using the breathing-orbital valence bond method,which allows the VB structures to have different sets of orbitals.Further valence bond calculations in a larger space of configurations,involving explicit participation of the σ response,with nine VB structures for the N state and 14 for the V state,confirm the results of the minimal structure-set,yielding an N-V transition energy of 7.97 eV and a spatial extension of 19.16 a0 2 for the V state.Both types of valence bond calculations show that the V state of ethylene is not fully ionic as usually assumed,but involving also a symmetry-adapted combination of VB structures each with asymmetric covalent bonds.The latter VB structures have cumulated weights of at least 18-26%,and stabilize the V state by about 0.9 eV.It is further shown that these latter VB structures,rather than the commonly considered zwitterionic ones,are the ones responsible for the spatial extension of the V state,known to be ca.50% larger than the V state.