We present an accurate and cost-effective method for investigating the accretion reactions between unsaturated hydrocarbons and oxidized organic radicals. We use accretion between isoprene and primary, secondary and tertiary alkyl peroxy radicals as model reactions. We show that a systematic semiempirical transition state search can lead to better transition state structures than relaxed scanning with density functional theory with a significant gain in computational efficiency. Additionally, we suggest accurate and effective quantum chemical methods to study accretion reactions between large unsaturated hydrocarbons and oxidized organic radicals. Furthermore, we examine the atmospheric relevance of these types of reactions by calculating the bimolecular reaction rate coefficients and formation rates under atmospheric conditions from the quantum chemical reaction energy barriers.